kienhow kindly setup ur wifi like this
-wireless mode N only
-channel bandwith 40MHz
-for control channel its depend on your area please use "Inssider" from here to check what is the best channel for you

may i know what device you use connect to wifi?
try change the channel to 1 and 11
if still same try updating the firmware

try both channel .. still the same
I am using dell laptop with build in (Intel® WiFi Link 5100 AGN adapter).

My router is just infront of my laptop...

which firmware u referring?
Currently I on 3.0.0.4.260..

Latest firmware here: https://www.asus.com/my/Networking/RTN66U/HelpDesk_BIOS/

You can already change VLAN for HyppTV connection but it needs to be wired connection. If you want wireless hypptv connection, you have to get Mikrotik for that.

Not possible, only wired. Try Mikrotik and ask bro soonwai for opinion. You can try wired connection through powerline. Have you considered that?

Simple. Check your 5GHz to see if it is 5GHz N or AC. I think yours should be 5GHz N on the N66u, and it is now 5GHz AC on the AC-1300UHP. Your laptop does not support 5GHZ AC. So, what you can do is to either get a WiFi adapter that can support 5GHz AC, which will be able to support your turbo speed in the future, or stay on 5GHz N.

Log into the router and go to the wireless settings and go to 5GHz and set it as 5GHz N/AC.

Yes, it should work, unless you need to set a too.

As I say, set your 5GHz as a/n/ac mix, not just n/ac. The TP-Link 5GHz in other house probably have mix of a/n/ac.

However, a is only up to a max of 54Mbps, so if your device can’t support AC, you won’t be able to enjoy the turbo speed.