GPON SFPs are a thing - it could be that the ISP is using those even with their official router, handing off all the GPON auth to the SFP and the router just se

GPON SFPs are a thing - it could be that the ISP is using those even with their official router, handing off all the GPON auth to the SFP and the router just sees this as a normal WAN port. It's actually quite a good setup, as it allows power users to swap the router with anything SFP-capable and provides a standard interface across the demarcation line.