One may go to your demarc if it’s outside so the service can feed back in to the distribution point

Sounds like you need to investigate how “most likely” that is on your attic.

I’d check the attic, then see if those gimmicky through the walls locators are on late night infomercials.

Or call an electrician.

Do you have any “blank” wall plates/cover plates in your house? On the walls or in the ceiling? If yes, check under those.

Per another comment, a tone generator might help. The “wireless”/probe end of the generator will work through walls of the cables are close to the drywall. It’s still a guessing game, but it might give you some clues.

Post some pics of the RJ11 terminations (take off the wall plate). As long as there are four twisted pairs, there’s a good chance you’ll get gigabit speeds over them.

Can you contact the builder? They might have a low voltage plan and/or remember the approx locations.

Have you tried following them with a magnet ?!

You need a tone and tracer, with a bit of knowledge how to use it for this type of house cable. The traceable field is very tight.