[OT] PC Network Help Needed Please

It's a DSL modem and the plug coming from the wall has a jack going into
the modem that's quite a bit smaller than the ethernet plug. There's no
ethernet cable coming from the wall. That is why I am confused. I don't
understand your instructions because I can't separate the DSL
instruction from the cable instructions and I have no idea what a "WAN
port" is. But it does sound like I need an additional (third) ethernet
cable....

Yes, you will need an additional ethernet cable. You should be able to
plug the new ethernet cable from one of the L1-L4 ports on the router to
the new computer. Set up the new computer to get's it's network setup via
DHCP and you should be set.

Dan