Hey @Cptuk! Thanks for this update. What’s an efficient wifi network in 2021 without some curveballs? It sounds like you are on the right track, and know what to look for here. Changing channel frequencies is a great call.
There are so many new routers I’m unsure about your precise model in the Archer series, however, if it has an “ax” mode and you have the option to choose 802.11 b/g/n wifi standard, I recommend choosing the 802.11 b/g/n without “ax”.
As you disabled the 2.4 Ghz network but still could see a 2.4 Ghz network available, I’m wondering if a beamforming type of router feature might be grouping wifi frequencies. Something that has personally worked for me in the past (not on a tp-link) was checking IP / DHCP features, Firewall/ encryption options, and SIP (packet inspection) related settings. Although I do not believe it to be related, here is our help center article on ports and protocols that might provide some helpful information.
Just a heads up; the above recommendations are only hypothesis, but we will always try to help where we can. Our support team also has the option of escalating to advanced support for more in-depth analysis. I hope you find a solution, and if you do, you are more than welcome to share it here.