https://openwrt.org/toh/tp-link/archer_c20i#tp-link_archer_c20i
I wonder if anyone knows what I need to do to get the 5Ghz working on the archer C20i
The wiki page doesn't mention anything about this but it doesn't appear to be working after installing OpenWRT.
While I have no experience with that particular device, the hardware should work. Set the regulatory domain (same for both radios, and correct for your location), configure the wireless parameters (ESSID, WPA2PSK/ CCMP, a good password - set a fixed channel (not auto)).
Then give it time to settle, DFS requires passive scanning for concurrent radar installations, before a channel may be used - this takes at least one minute (more under certain circumstances), so let it sit a good 10 minutes, before declaring failure. Then check with an external device (phones are nice for this, but any laptop/ desktop will do as well) if you can find your AP.
There was an older forum post from a long time ago stating that 5Ghz was not working
https://forum.archive.openwrt.org/viewtopic.php?id=57446
I don't know if this issue has been resolved.
That was 8 years ago, mt7610e should be supported by now (but again, I don't own that device, nor any of the older mt76 supported chipsets myself - but the c20i build recipe explicitly pulls in kmod-mt76x0e, which is a good starting point; nor does the device page on the wiki have any scary warnings about the wifi side, it's not going to be a great device, but it should show life signs on both bands).
Ok I think I've figured out what it is you can't seem to use channels in UNI-III
Maybe this is something to do with the lack of DFS support. Channels 36-48 seem fine trying to use 149-161 you have all sorts of issues.
I don't see this on my netgear router but I think they have an opensource atheros driver.