If this were my router, I think I would delete the wireless configs and start over...with a very simple configuration.
I would use LuCI instead of UCI...less chance for errors, and if your forget to do a "uci commmit", the changes won't apply.
A simple Save, then a Save and Apply (buttons included) just makes it easier IMO.
I use a C7 V2 with 2 guest/IoT networks.
A 2.4 GHz interface with WPA2-PSK AES, and one of the non-overlapping channels (1, 6, or 11).
A 5 GHz interface with WPA2-PSK AES and one of the channels in the range of 36 to 48, or 149 to 161.
Make sure the client devices are using the correct WiFi credentials...for each wireless band.
WMM enabled on both.
No 801.11r or 801.11w enabled.
Use the defaults for the remaining settings.
As plain vanilla as you can make it...
Then test, and build it up from there.