[CursedSilicon]

I tried for years to do similar when I lived in Australia. Though with a PCI/PCI-E card

ZyXEL if I remember correctly did make an ADSL2+ at the time PCI-E card. Literally just a DSL modem wired to a Realtek 8139 NIC. You could slap it in a Linux (or BSD, or Windows) PC and just use PPPoE to connect to the internet

Naturally it was impossible to order the damn thing and I never got to realize my dream of an "all-in-one" DSL Linux router.

After moving to DOCSIS (Cable) internet I ran into the same confusing problem. "Thankfully" with Fiber everything is just ethernet (more or less) now. But it was an infuriating time in the 2010's