Readjust or reposition your antenna while monitor the signal Strength readings.
What is your area and state?

Centralised antenna system (MATV) must be upgraded to support/receive digital TV signals due to technical reasons. Otherwise you won't receive any signal at all.
Malaysia's analogue TV broadcasts has been switched off end of last year, so it's no more already.

Or else, you'll have to use your own UHF antenna.

You can consider get antenna models like this, which is known to be pretty good:
https://shopee.com.my/Digital-Antenna-UHF-H...38079.313253815
https://shopee.com.my/VT333-MR-U805OD-UHF-H...8079.1369423471
https://shopee.com.my/MYTV-TD-5E-ANTENNA-TV...99971.941931668
Since the antenna is small in size, basically you can just place it anywhere near the TV as long you get a good reception, so not necessary must put outside of your house or something (depends on location too).

Or, you want to buy an outdoor one also ok:
https://shopee.com.my/ME-Uhf-HDTV-Outdoor-D...38079.435769904

As for the cable, please use RG-6 cable instead of 3C-2V or 5C-2V cable, for a better overall cable performance = better signal reception.
RG-6 coaxial cable is the same cable type used in satellite TV, such as Astro.

I believe you've tried Auto Search the channels, right?
If you use your centralised antenna and you experience this, inform your building management about this.

If your building management isn't helping, then consider buy own UHF antenna to get things done on your own.

I've tried with my friend's TV in Metro Prima (apartment) with a cheap antenna from Mr DIY, I think within RM6. It has magnetic based so you can attach to the window and voila all channels appeared.

Hmmm, what an embarrassing OUG Parklane service apartment. Next time don't buy that apartment.

Today, there are 2 frequency muxes being used for Malaysia's Digital TV broadcast nationwide.
Each frequency mux can accommodate a few number of TV & radio channels.

For Klang Valley region, the 2 frequencies are 650MHz (UHF Channel 43) and 666MHz (UHF Channel 45).
The 650MHz mux mainly carries RTM channels (TV1, TV2, Sukan RTM, Berita RTM,...), while 666MHz mux carries Media Prima and other channels (TV3, 8TV, TVS, Suke TV, Awesome TV...).

So, the definition of "Rtm low frequency" that your building management staff used, is actually referring to 650MHz mux, while "Media prima high frequency" is referring to 666MHz mux.

Basically what your building management is telling you is, they did not upgrade the building's centralise antenna system (also known as Master Antenna Television, or MATV) to support both 650MHz and 666MHz frequencies.
Somehow, they've only installed/configured 650MHz frequency mux, that is why you're only able to watch RTM channels when you connect your TV or your digital TV set-top-box to your room's TV wall socket.

A possible reason why the management staff gives you the excuse of "Building sini anttena is analog not digital" might because they don't intend to spend anymore money to upgrade/support additional frequency of 666MHz.
The common reason why some building managements out there don't want to upgrade/maintain centralise antenna system to properly support digital TV broadcasts is because it is not cheap.
It could costs thousands or few tens of thousands, or more, depending on the complexity of your building's MATV system.
Money reasons alone is enough to justify whether is it worth the investment or not, considering how many residents in that building block actually care to watch digital TV broadcasts and bother to raise complain about this.

In this situation, the quickest solution you can consider, is buy an indoor UHF TV antenna, or a small-size outdoor UHF antenna for a better reception, if you don't mind the appearance of the antenna.
Use that TV antenna and connect it to your TV or your digital TV set-top-box, rather than connecting to the wall's TV socket (like what you're doing now).

I checked on Google Maps, the closest TV transmitter from your OUG Parklane Service Apartment, would be Bukit Sungai Besi which has a line-of-sight distance of approx 8km.
To receive the TV signal from this transmitter, aim your TV antenna direction towards East.

The second and third closest TV transmitter would be KL Tower and Gunung Ulu Kali (this is at Genting Highlands) which has a line-of-sight distance of approx 10km and 42km respectively.
To receive the TV signal from either this 2 transmitters, aim your TV antenna direction towards North-East.

Reminder: Line-of-sight distance is NOT the same as driving distance

Usually general rule is, if the line of sight distance from your receiving location (i.e. your apartment) is not more than 30km, then indoor TV antenna type should be sufficient.
But that also depends on the position/location of your apartment's unit, especially the direction of your window or balcony.
If it has a clear line of sight without big obstructions, then your chances of being able to receive stable digital TV reception will be higher. The higher the floor you stay on, the higher the chance of able to receive strong/stable TV signals.