I haven't had to do this specifically in the past outside of unique situations (usually with Juniper/SonicWALL/Palo Alto) of which this is one.
XFinity requires use of their gateways in order to connect but unfortunately the new XER10 does NOT support bridge mode. This means either deal with the issues that come along with double NAT or find some workarounds. In this case, the thought is to run the single port from their gateway to the WAN port configured on the OPNSense router and set a port forward of 1-65535 to a static IP that will be assigned to it and then manage everything from OPNSense, effectively creating a bridge mode. I know it won't function exactly the same but I would think it should get pretty close.
Supposedly XFinity may remove the EPON requirement in the future and this won't be an issue but figured I would see how this goes for now. Thanks in advance for any advice.