Unifi Security Gateway MTU & MSS Clamping
Here is how to possibly fix a Unifi Security Gateways to work on a PPPOE connection.
On the Unifi controller, set the security gateway to enable “MSS clamping” and set the size of clamping at a custom size of “1406”.
If that alone does not fix the problem, you might need to also set the MTU size to 1480
Unfortunately this can not be done from the GUI and will need to be performed through CLI. SSH onto the gateway and running:
set interfaces ethernet eth0 pppoe 0 mtu 1480