@tom_tom im not familiar with the USG but from what I can see, by default it expects to be connected to a modem and that it will make the PPPoE connection. That isnt going to work since the Hub one cannot be configured as a modem only (bridge mode).
Two alternatives I can see :-
1) Set the USG Wan to DHCP so that it gets an IP from the Hub one. You will also need to change the LAN subnet on the USG from 192.168.1.x to something different to that of the Hub one , I suggest 192.168.0.x.
2) Set the USG WAN to static IP, with an IP of 192.168.1.1 and gateway of 192.168.1.254 , and disable DHCP.
Disadvantage of 1 above is that you have a double-nat situation which may cause problem if you need port fowarding for games , voip etc
With 2 , I'm not sure if it will work, but I'm hoping that it effectively turns the USG into access point mode.
For both of these , I would suggest disabling the wifi on the Hub one.
If 2 works, then you could possibly experiment with using the DHCP server on the USG instead of the Hub one, since it's more configuarable in terms of DNS servers I would guess.