I am using C2600. So how? Still supported?

Check your WiFi setting for "Group Key Update Period". If it is a number other than 0, when the key is changing, you will experience this effect. Change it to 0, the key will not change, so you will not get this effect.

Try disable IPv6 completely and only running on IPv4.

See if it is Unifi IPv6 problem...

Here are some of the things to try out:

1. Change the location of your WiFi router. Maybe there are too many inteferences around. Don't put close to TV, desktop computer, refrigerator, Hi Fi system, cordless phone, ...

2. Your connecting device (laptop or handphone) should not be too close or too far away from the WiFi router. Test within 2-5 meters away. Too close, especially less than 1 meter away, will be very slow. Too far, the signal weaken and makes it slow too.

3. All antennas of the router should be in upright position. There might be people in the internet telling you to angle the antennas in funny funny position. I have tested myself with my TPLink, the position with the best signal is all antennas upright.

4. Test your local WiFi speed. You might need 2 devices to perform the test by transfering data from one to another. A free tool to test is LAN Speed Test by Totusoft. If your local WiFi speed is OK, the problem might not be on the WiFi but on something else.

The rt-n12hp has 9 dbi antennas, while the c1200 has 5 dbi antennas.

Higher gain antenna can reach further but has narrower coverage field. It is good for fixed WiFi connection (the device won't move around, such as desktop, printer, smart TV, etc.) that are some distance away and located within the coverage field. If the connection is moving around (smartphone, laptop, tablet, etc.), you will get good connection at certain area, and poor connection at certain area.

Lower gain antenna has broader coverage field but shorter reach. It is good for both fixed and mobile WiFi connections, that are located not too far away.

The main advantage of c1200 is its 5 GHz band. You can enjoy fast speed with wireless-ac connection.

For modern smart TV that supports 5 GHz wireless-ac and high resolution display (full HD, 4k, ...), you need the wireless-ac WiFi for smooth and lag free high resolution video streaming. C3150 or C3200 are good choices. I use C2600 (still sold in the market, but already taken off from TPlink Malaysia website), so far so good. My router is at 1st floor and my TV is at ground floor, about 20-30 feets apart.

Look for these 2 keywords in the product: MU-MIMO, beamforming.

WiFi extender will slow down the maximum WiFi speed to half for devices connected to it. However, its purpose is to extend the coverage for far away devices, which WiFi speed will not be too much if connected directly. So, it still able to improve the WiFi speed for those devices.

Example:

Far away device support 300 Mbps Wireless N. Max speed is 300 Mbps. Due to long distance, actual direct connection speed is only 12 Mbps.

With WiFi extender, maximum Wireless N speed for that device connection is 300/2 = 150 Mbps only. Actual speed connected to the extender which connect to the router could be 75 Mbps, still much better than the 12 Mbps direct connection.

So, if distance is near, direct connection is faster and stabler. If distance is far, the extender can help to boost the reach and the actual connection speed.

For review of TPLink WiFi extender, can refer to the link below:
http://voyager8.blogspot.com/2017/07/boost...link-re450.html

TPLink C1200 only has 128 MB RAM.

You should get C2600 which has 512 MB RAM.

C3150 RAM is 256MB.
C3200 RAM is 256MB.
C5400 RAM is 512MB.

For normal rod antennas, the WiFi signal travels upward. It is better to put the router downstairs to have better coverage to both floors. If the router is upstairs, coverage to downstairs will be weaker.

I also have my router upstairs, end up I use a range extender at downstairs to boost the wifi stability and speed downstairs.

Correction: sideway, more upward than downward.

It looks like this:

I doubt you can use powerline for hypp tv if you already use it for PCs, because their network are separated.

Unless it is possible to implement 2 different networks of powerline within the same wiring system.

As a quick fix, you can get a SUM Digital Timer to daily off the router at one time and on it back at another time.

Even if the router is functioning well, I use it to let my router have some rest once per week too. The timer is very flexible to have up 10 sets of on/off timing which can be in minutes, hours, days or weekdays.

That one is for Streamyx instead of UniFi...

For ping to work, you need to open the ICMP port. Some router disabled it from external by default.

For other to know your public IP, you can make use of dynamic DNS service, and set up your domain name. Unfortunately, most dynamic DNS service nowadays are either not free, or need China email to register for an account.