Hi there, did you ever solve your USG issues?
I believe I'm encountering something similar.
These are the steps I took and then got stuck...
1. Set modem to Bridge mode (this is where DHCP was being provided)
2. Plugged in USG (WAN port) to modem and then (LAN port) to my switch
3. Reset the USG, adopted and updated from my existing Unifi Controller
4. ....
And this is where I'm getting stuck. I can't seem to get internet connectivity on devices via LAN or Wireless, which all comes from that 1 switch of mine. I can't seem to see any errors when updating the USG, so I'm guessing there is Internet.
I have also set the DNS servers (which don't get set automatically?).
Interestingly, from my desktop, I renew DHCP, get an IP and see the gateway (192.168.1.1) and the DNS servers I input....but still no internet connectivity.
I can get to the USG admin page but I can't log in with the default credentials OR with my online account.
=(