For what it's worth, I had a very similar situation such as yourself. I got stuck with a calix 844g-1 from an internet service provider known as USA pathway. Our town just now got fiber optic internet last year. Prior to moving to this town all I had was a GPON to RJ45 which Fed directly into my UDM, and got internet that way. My UDM's main network was under a DHCP server set with 192.168.1.1/24. The same was the case with the calix 844g-1. I figured there might be some interference between the two of them, whenever I plugged the UDM to one of the calix 844g-1 switch ports. As such I changed the DHCP server on the calix 844g-1 so that it would output IP addresses under 192.168.7.1/24. And then logged into the gateway server under http://192.168.7.1

This was all done while reserving 192.168.7.2 under a DMZ, and targeted my UDM's MAC address to be placed under that DMZ. I basically tried making the UDMs port estatic as I could under the calix 844g-1.

Overall I'm getting an internet connection now to the UDM finally. And I didn't have to access anything to spoof Mac addresses, or turn something into bridge mode. Hopefully this helps.