It seems like your current configuration for radio0 does not has HT mode enabled at all, which means the 5Ghz band will fall back to sluggish 802.11a speeds. Do note that if you enable wireless.radio0.htmode=VHT80 from the command line or by editing the config file, it will be remove if you do a save&apply from the web interface.
I haven't tested with different channels (and can't right now) but the following works for me. I'm not sure if the country code has any relevance, but I'm in Finland so I put it accordingly.
Thanks for you reply! I wasn't aware that changes in the webinterface overwrite some settings again so now I am doing all changes via the uci command line. I tried your values but it didn't change anything. I set the htmode to VHT80, channel to 37, country both US and DE.
I also set the log level to 1 but I still can't find anything in the logs that seems relevant. I enable the interface in the web gui but the state stays at "Wireless is disabled or not associated". What options do I have to find out what is going on? Is there a guide somewhere on debugging these issues? Like how to see if the device driver can even properly communicate with the hardware? Or is that a given if the 2.4 Ghz interface works?
Thanks for your help!