Found this through Google so will post my experience as a reference for anyone with a similar setup to me:
Ubiquiti USG (Cloud managed)
Draytek 130 VDSL modem
First, I upgraded the firmware on the Draytek 130 (connect via ethernet cable, set your computer to 192.168.2.2 then go to 192.168.2.1, username and password are admin/admin). I then set the PPPoE/PPPoA settings to Protocol - PPPoE (under Internet Access - PPPoE/PPPoA).
I then reset my IP and connected to the USG over the LAN. If you have the Cloud managed version make sure you log into your cloud account first, under Settings you can get the unique username and password that you have to use.
Once logged on to the USG change the connection type to PPPoE. I use the following username/password combination: