Issue with Modem Mode for a FTTP Hub 5x

I'm trying to setup a UniFi Express router, and I can't seem to establish an internet connection in modem mode. I've tried variations along the same theme, router resets, different ports, disabling Firewall, DMZ, DHCP, ensuring one cable is plugged in, and attempting modem mode directly to my laptop. In every case, no internet connection.

The first oddity I've noted is that the public IP being assigned is the same in router mode and modem mode.

The second oddity is that I've tested modem mode a few days ago, during a time where there was a fault and I couldn't connect to the internet in router mode. However, at the time, I could establish an internet connection by having modem mode connected directly to my laptop (this is what drove me to purchase my own router). A few days later, now the fault is fixed, I have the same problem in reverse. Router mode works, modem mode does not. 

Short of obtaining another 5x router for a new MAC address, I'm not sure what else to try. 

I'd appreciate any insight, and thank you for your time.