Need help switching to customer owned router

I have my ubiquiti cloud gateway-fiber (UCG-Fiber) connected to my XB8 through bridge mode. My location has Fiber available using the ONU device. I want to be able to connect my UCG-fiber directly to the ONU device using Ethernet cable without using the XB8. I’ve seen that Xfinity now allows this to happen but when I reach out to support they think I’m trying to connect my own modem. Any help with this?