> But it just doesn't work like that for DOCSIS or GPON where the cable modems or ONTs these days do much more than just media conversion - SIP, PPPoE, IGMP, etc. even if they don't do Wi-Fi (so ISPs don't call them "routers" - except SingTel, which uses "ONR" to distinguish these units because they are in fact routers for IPTV and SIP).
At least in Finland the norm is that you can use your own DOCSIS modem from any manufacturer, you just tell the ISP your modem's MAC address.
I'm not a subject matter expert by any means, but the 8311 Community Firmware group has made great strides on their SFP+ ONT FWs. The majority of users are using their WAS-110 implementation for XGS-PON to ‘bypass’ the ISPs ONT/Gateway device, in the states there's a lot of users doing this on AT&T Fiber, but the Discord community has a large global following with users around the world reporting success. They have some users on GPON, its a bit of a different bypass but people have been successful. I run a bypass directly into my UDM-Pro using a WAS-110.
For me, Comcast only allowed a few models. Because it requires full control. They uploaded a Comcast firmware on it. And they warned me that after that the device is basically stuck to only work with Comcast. This was around 2017.
That sounds odd - I wonder if it was a relic of your local infrastructure. I used my own DOCSIS modem with its own firmware on Comcast around the same time. It was a generic Arris Surfboard.
Comcast's list of approved devices includes (or used to at least) some of the more common Arris Surfboards; I also used one when I had Comcast service. You probably also used one that was 'compatible'.
At least in Finland the norm is that you can use your own DOCSIS modem from any manufacturer, you just tell the ISP your modem's MAC address.
Not for GPON, though.