I think I'd configure the 5 Ghz wifi as client to my main router, but don't know how to finish the setup.
afaik:
network -> Wireless, then choose the wifi network generated by the 5G radio, Edit.
Go in Interface Configuration tab, in the Mode choose "client", save and apply, then enable that interface.
Then from network -> Wireless you press Scan and it should show wifi networks you can connect to.
Consider that It's a while I've done this so I might be mistaken.
As a final step, I'd like to also use the wired Ethernet to get access to the main router with its DHCP. But this step is not as important as the previous one.
In the radio settings, Interface configuration, Network line, choose Lan (which should be default for ethernet ports of local network) for both radios.
Then from Network -> Interfaces click on Edit in the LAN interface, there you can give the device a static address (or not) and down in the DHCP Server tab you can select the "disable DHCP for this interface" so it will not interfere with the current DHCP server.