Set your 5GHz to run at 80MHz. Not 20MHz, not 40MHz, not auto, not 20/40/80MHz but instead set it to 80MHz only. Don't ask me how, read the manual of your router
Also, if you can afford to pay UniFi 800Mb/s, you can afford to buy a better router
All stock routers from TM/Maxis/Time is crap

Get at least ASUS RT-AC66U B1
Like you, I'm also using UniFi 800Mb/s. I can get 600Mb/s on ASUS RT-AC68U, RT-AC86U & GT-AC5300. RT-AC66U B1 is basically the same as RT-AC68U so you should be able to get at 600Mb/s on WiFi as well
Below is a screenshot of the Speedtest on WiFi



No
You can only reach 300Mb/s on 2.4GHz if your WiFi receiver is 3x3 & running at 40Mhz

Most devices only supports 2x2 & that limits you to 150Mb/s which translate to actual speed of 90 ~ 100Mb/s

1. If you want full 300Mb/s, you need 5GHz running at 80MHz
2. Dump that WR1043ND. That's a very old router. My suggestion to at least get an ASUS RT-AC66U B1

What's confusing about that?
The ISP is actually giving you the speed that you subscribed, but only on Ethernet cable. No ISP in the world can give you 100% of the speed that you subscribed on WiFi because that is impossible as it depends on several factors

Let's say the ISP has already configured the best wireless settings for you, when they test on their phone that supports wireless AC 2x2, they can get full 500Mb/s
But the customer is using a cap ayam phone that only supports Wireless N 1x1 that only maxed out at 72Mb/s, is that the ISP fault?
I use chap korea branded phone wo...
That one you need to figure out yourself
You need to find out which channel is less congested then use that channel

wah lau bro how to find out leh??? man tau all this things??


For ASUS routers, you can just use the built-in "Wi-Fi Radar" tool to find out which channel is best for you
Not sure about other brands, you'll have to figure it out by yourself

Here, let me help you:
https://www.google.com