@Genux I stuck with the older firmware, sorry I can't be of more help. To be honest i forgot about it until you mentioned it, so I may try again. Wasted too much time last time, and need the patience to get into bug fixing mode. 

@Genux so, I had another try with the newer firmware. Factory reset, installed new firmware, then configured the wife network etc using an ethernet cable from my laptop.

Last time I did this all my devices were connected using the 2.4Ghz connection, since then and due to so many of us being at home during lockdown, I've moved a lot of the laptops and heavily used devices to the 5GHz channel, for better resource sharing. 

Results were same as last time on the 2.4GHz channel, it simply does not work, I can't connect to it using my iPhone, iPad, TV's won't connect, I've got an older network extender that only uses 2.4GHz, that wouldn't connect either. Nothing had changed, I restarted all these devices, using DHCP on the routed... I even did a network settings reset on my iphone and it still wouldn't connect to 2.4GHz. I have a lot of home automation too, and that is all connected to the 2.4GHz channel... same result, it all started flashing saying NO CONNECTION. The settings on the router are all for max compatibility, so DHCP is used, there are no blocks or black lists or white lists... it's as compatible as it is ever going to be. 

This time though, unlike last time, I also had devices connected to 5GHz connection. They were absolutely fine, in fact, they performed better and the connection seemed more stable, however my house can't run without the 2.4GHz frequency as too many of my wifi devices connect using that frequency and aren't compatible with 5GHz. So i ended up doing what I did last time and have downgraded the firmware. I note the latest firmware is now listed as 'BETA' on the TP-LINK website, I don't recall it being listed as BETA last time, so maybe an acknowledgement that it's buggy. Certainly for 2.4GHz it's useless.