Thanks. I have an ISP provided media converter with my own router behind that, using the correct VLAN was enough to get it working. I thought those media converters were pretty dumb devices but it seems they are not.

They are not dumb but are very standardized. Unless they are issuing and verifying device certs you can almost certainly use your own PON equipment with very little effort.

If they are using certs youd have to extract it. The vast majority of ISPs don't bother or care.