I set up a Unifi system in Thailand that has to be downstream (DHCP) from an ISP provided router. Both Unifi and ISP device want 192.168.1.x and want to be the DHCP server and there’s no easy way to tell Unifi that “WAN” is 1 hop away.

I agree with the poster that if you are doing anything that is not a defined happy path for Unifi, it is a freaking nightmare and will likely involve rebuilding, resetting and readopting several times.

Mine was perfectly happy to automatically set up on something else when it was downstream of another router, and subsequently moving it up to be the primary router was equally painless.

YMMV, though I suspect newer software on the device might be making a difference here.