Where do you live?
What kind of antenna are you currently using?

If not close to JB u need external aerial,and in many places further from JB only Channel 8 and Vasantham remain while the rest always go missing

able to get the malaysia channel very clear
Instead of "very clear", the main thing you should concern about when it comes to digital TV broadcasts is, whether you experience intermittent screen pixelation/blackouts (means unstable signal) or not. If yes, re-position your antenna. If no, then you're good to go.
The proper way is, you monitor the signal readings (especially Signal Strength) while you readjust the antenna position, rather than just to "see whether you're still able to view the TV channel or not" (this method only make sense for analogue TV broadcasts, which has ceased many years ago).
Yes.
Considering the line-of-distance from Kulai to Singapore (<45km), choose the 5 elements (5E) outdoor antenna is good enough. Make sure to aim the antenna towards south position (which is towards Singapore).

To do manual tuning, enter the following frequency or channel number.
Monitor the signal reading as you adjust the antenna.
If you see 0% signal reading, adjust the antenna position until until it reaches the highest signal readings possible (Strength %), then press Scan (or Search).

Here are the frequencies for Singapore digital TV broadcasts

Frequency (in MHz)	Frequency (in KHz)	CH / Channel	What channels does it have?
538MHz	538000 KHz	29	Channel 5, Suria
554MHz	554000 KHz	31	Channel 8, Vasantham
570MHz	570000 KHz	33	CNA, Channel U

This has to trace back to an era when analogue TV broadcasts still exists (before 2019).

Analogue TV reception can be greatly impacted by even the slightest movement of the outdoor antenna. As Kulai is quite far away from Bukit Batok mast, a higher antenna has to be installed to ensure clean, clear analogue TV reception of Singapore channels. For the case of Channel U (analogue) and Suria (analogue), you have to also consider co channel interference from Gunung Ledang, which in Kulai, the signal is directly receivable and so careful installation has to be done to reduce the interference.

However, digital TV switchover have made antenna installations less complicated, as there are no more "snow", "ghosting" on digital TV. You either receive the signal, have pixelation (as a result of weak signal) or get nothing.

On a Singapore TV show, it shows that with correct antenna size/selection and also height (20m+), one can receive digital TV signal from Singapore in Muar clearly, exactly like how Singaporean viewers can receive the signal.

Lol, what makes you think they would "overdo" things to receive Singapore channels? 

Btw are you able to receive Singapore channels now?

Since 5 element antenna will be enough so I wonder why there are people installing complex structure one with many elements
Those antennas around your neighbourhood is already there since don't know when, right?

Back then during analogue TV era, I wouldn't rule out the possibility where people would just recommend you to use UHF antenna with a lot of elements, simply because it "can receive signal from far far away", therefore "the picture will be very sharp".
There is a misconception that antenna with more elements always equals better reception. This is one of the common sales tactics, since antenna with more elements costs more, hence it's easier for installer/electrical shop uncle to upsell it to you, hence more profit margin for them. Same goes to signal boosters as well.
But in reality, since the line-of-distance from the transmitter tower Kulai to Singapore is less than 45km, and in today's context, Singapore's digital TV broadcasts uses a pretty robust modulation of 16QAM (meaning the signal is more robust and doesn't degrade easily through obstructions, as opposed to Malaysia's 256QAM), hence opting a UHF antenna with a lot of elements for your case isn't strictly necessary, as a simpler, smaller antenna like 5E could have done the job just fine.

Example: https://shopee.com.my/Digital-Aerial-indoor...5714.8716544271

Tip: Only if you experience weak or unstable signals due to some local obstructions, then only you use signal booster (e.g. USB-powered ones). Avoid using one if you're able to receive strong and stable signal already.


If you use UHF antenna with more elements (like 14E or more), not only the overall antenna size will be bigger/longer (and slightly expensive), but it'll be a little more challenging for you to get a strong and stable signal as you perform adjustments to the antenna position (compared to 5E).
That's because as the number of elements increases, the antenna beamwidth will become narrower, more focused and sensitive, making reception of distant TV transmitter (e.g. more than 80km) to be possible. But do note that you'll need to fine tune or aim the antenna position properly towards the direction of the TV transmitter tower, as even a slight misalignment of the antenna position could result a big drop in signal readings (or even zero signal).

This is especially true for those who want to try receive Indonesian digital TV channels (from Batam) in Johor Bahru area. Plus, the output power of Indonesia's TV transmitter tower from Batam is not as high as Malaysia and Singapore, hence placing the UHF antenna as high as possible + UHF antenna with a lot of elements + signal booster is necessary to get a stable reception.

I have not bought the outdoor antenna yet, I tried to search the Sg channel with indoor antenna on 2nd floor but nothing
I assume you're living in a landed house?
The position that you place your antenna, you place it near the balcony or window? Can confirm whether is it facing around southeast? Any local obstructions (e.g. trees, buildings) blocking the antenna's view towards southeast?
Can show us a pic of your indoor antenna, or send the link of where you bought it?

I tried to place the antenna link below on the iron grill of 2nd floor window but only can get the MY channel
That window you're referring to, is it facing towards southeast?
Do you see any big building blocks or big trees or big hills that is near to you that may be blocking your scenary view from the window? Need you to confirm on this before advising further.

If it's okay for you, can take pic of the view of your 2nd floor window?

I have the TV antenna wall socket at my house but don't know where it leads to yet, logically we can buy the 5 element outdoor antenna and connect with it right?
Do you see any TV antenna at the rooftop of your house?
If not, do you see a cable connection at the rooftop of your house, which may lead down to the TV antenna wall socket of your room at 2nd floor?

If worse come to worse, buy new RG6 wiring, buy 5E antenna (or opt for 14E antenna if you're worried), lay the new cable from your rooftop down to your room, and check if you're able to receive Singapore channels.
If not sure, you can hire a technician that does TV antenna installations to come over and do it for you.
If you really opted for this option, you might as well target for being able to receive Indonesian channels also hahaha...

I tried to place the antenna link below on the iron grill of 2nd floor window but only can get the MY channel
That window you're referring to, is it facing towards southeast?
Do you see any big building blocks or big trees or big hills that is near to you that may be blocking your scenary view from the window? Need you to confirm on this before advising further.

If it's okay for you, can take pic of the view of your 2nd floor window?

I have the TV antenna wall socket at my house but don't know where it leads to yet, logically we can buy the 5 element outdoor antenna and connect with it right?
Do you see any TV antenna at the rooftop of your house?
If not, do you see a cable connection at the rooftop of your house, which may lead down to the TV antenna wall socket of your room at 2nd floor?

If worse come to worse, buy new RG6 wiring, buy 5E antenna (or opt for 14E antenna if you're worried), lay the new cable from your rooftop down to your room, and check if you're able to receive Singapore channels.
If not sure, you can hire a technician that does TV antenna installations to come over and do it for you.
If you really opted for this option, you might as well target for being able to receive Indonesian channels also hahaha...