EPON Member-Owned Router: need ONU provisioning for Ubiquiti UDM Pro

I have Xfinity Fiber (EPON, 1 Gig) with an XER10 gateway and a Nokia ONU (NKFN11AEL).

I would like to use my own gateway Unifi UDM-Pro.  I see other people on the forum have been able to get this working but having their ONU provisioned?

I moved the cable from the xfinify gateway to the UDM-Pro with no luck (as expected).  I reached out via chat and phone, and was told at both places that you can't use your own gateway.

Is this true?