It very strongly depends on which ISP they have. There's a few that make it easy. There's a much larger number that can be hacked by a competent pc person (which I've done). There's also a small amount who have worked to make it impossible / hard to do, and don't have any public info on the process.

My ISP is att fiber, and all I had to do was change the vlan id on the outgoing side and match the ip settings to make it work. I used the guides from https://pon.wiki/ to do it, and the discord is also incredibly helpful.

Of note, this used to be impossible / very difficult so you'll still find forum posts saying it can't be done. However, a couple nerds have changed that over the past two years so make sure your info is up to date before deciding it can't be done for your specific setup.

I checked this route but fiber modem are currently rare. There are only few WiFi 6/7 routers which accepts fiber. My ISP on the other hand is quite friendly. They initially provided me with a fiber modem, which sucked as it was quite old, so I told them to give me a simple modem as I have my own ethernet wifi router. They replaced it the next day.