When trying to change WiFi settings on my TP-Link TD-W9970 v4 I got “Error code:1 Internal error” (and one button saying “OK”; it is not OK!). The TP-Link forms indicate I am not the first, but the TP-Link support people haven’t given any useful advice. Taking a backup of settings, resetting, and restoring the settings DID NOT fix things… just a waste of time.
Eventually, I stumbled upon the solution: I enabled the “Guest Network”.
Then I could change WiFi settings, especially the way the default “auto” settings are for a 40MHz bandwidth on an auto-selected WiFi channel (which is not helpful for managing multiple access points and a Zigbee network). The Guest Network can be turned off again afterwards.