Dynamic Frequency Selection
Dynamic Frequency Selection is a channel allocation schemes specified for wireless LAN, commonly Wi-Fi. It is designed to prevent interference with other usages of the C band frequency band that had predated Wi-Fi, such as military radar, satellite communication, and weather radar. It was standardized in 2003 as part of IEEE 802.11h.

In the US and other countries, some of the Unlicensed National Information Infrastructure (U-NII) bands are used by radar systems. Wi-Fi networks operating in those bands are required to employ a radar detection and avoidance capability. I think this is not applicable here in Malaysia. We are still on standard ISM bands.

The  Transmit Power Control (TPC) goes together with DFS to avoid interfering the radar by reducing the power.

I think you can have both disabled if there is such feature in setting

Have you ever installed any apps to check if there is other signal within your operating WiFi channel?

Indeed, you can pick 802.11ac only for 5GHz for starting. If you face hardly connected or frequently disconnected, just stick back to 802.11n/ac only. From what i can see, you are using tm stock default router because wifi name come with @unifi, you may refer from my past replied how to upgrade the firmware by using recovery mode, for temporary usage before you find the best deal router soon. i know this can make it confuse a bit for you. Just make sure make strong a password, not using a number only. Old timer tp-link router, 5GHz, usually support channel 36 until 48 only. DFS channel you need to try and error, if your place too much helicopter and aeroplane playing around in your area - flight live view , you very hardly to use these channel. So, you need try one by one channel if want to use DFS channel, not all router support DFS channel. Some of router that support these channel cannot use it properly, not functioning very well. Only certain of its got stable by using these channel, depend on what hardware specs of that router too. Nowadays, many router support 5GHz channel from 36 until 161/165 with channel width 80MHz. Only certain router that support channel width 160MHz.

That is standard speed usually for wifi 2.4GHz especially if you're using tplink router, depend on specs of your devices too. Bare in mind just connecting direct Lan cable only if you're using desktop, if using laptop at same place better using direct Lan cable. Just use wifi when need moving to other place and there is no way for wired.

Nope, can you recommend me the app so I can check?

You can put any widest band if you have no interference from any other WAP like from neighbor or your other WAP. I'd leave the 2.4GHz band for guess and old devices.

For the 5GHz band , it is different story because you have more channels. Unless your device can do link aggregation, you can only use either band only. So, no 1200Mbps total 

With the overheads involved, AC1200 should get you connected at around 867Mbps. AC1200 Wireless cards/adapters are priced affordably too.
That's good enough to cover TM 800Mbps , 500Mbps and 300Mbps plans.

If there are other isps offering 1Gbps plan above, you need to find better routers or don't forget higher end wireless adapters/cards such as AC1900, AX3000 or better to attain such speeds.

AC1200 should be enough for now to cover upto 800Mbps wirelessly for ISP plans in Malaysia for time being.

Don't think the calculation speed works that way. I was on TIME 1Gbps plan, was using AC1750 router, and AC1200 adapter on my PC. The most I can get is 290mbps. AC1750 is more recommended for speed upto 500Mbps. Not sure for higher speed though.

Don't think the calculation speed works that way. I was on TIME 1Gbps plan, was using AC1750 router, and AC1200 adapter on my PC. The most I can get is 290mbps. AC1750 is more recommended for speed upto 500Mbps. Not sure for higher speed though.

What is the connected speed for your wireless client? You can look it up under connected Wi-Fi status of your OS.
If it's 867Mbps or so and you are still getting very slow speed on your local based speed tests it means your ISP is at fault.

AC1750 router is good but it's hard to find matching AC1750 wireless adapters out there. Even it there is, you'll be paying a very high price for them while they're rare.

My WiFi adapter is AC1200, which by your logic should have 867Mbps. My ISP has no problem, because when I speed test using LAN it can go up to 900++ Mbps. Again, my point is, AC1200 is not the right router for Internet Package up to 800Mbps. It is only suitable upto internet package of 300Mbps