Or, get Internet service that includes a modem/ gateway with an Ethernet
port. This is probably the best answer in the long run.
Unless that modem/gateway is under the control of the ISP, in which case
you're fundamentally inviting your ISP onto your local network, IOW into
your private home.
[ Tho, of course, you can probably arrange to confine that "local
network" such that it's only connected to a single other machine which
you do control and which you use as your own local router. ]
So, you designed, built, and programmed your "single other machine" using
machines that you designed, built, and programmed; which in turn you
designed, built, and programmed using machines that you designed, built, and
programmed; etc.; all sourced from raw materials?