Wife and I downloaded to play some Zero Hour for childhood nostalgia, but she is getting the Serial Already in Use error,

both of us are on out own Origin account, with our own copy of The Ultimate Collection, with Shockwave 1.20.1 downloaded onto clean installs of Zero Hour.

Zero Hour works for us, and we can play shockwave skirmishes, but she cannot utilize the network for any reason.

All my searching for playing the game online seems to only come up with outdated links or instructions for the previous servers. How do we play nowadays?

try changing one of the serial keys and see if it still does it.
it's simple to do.
open Regedit on your computer
navigate to
HKEY_LOCAL_MACHINE\SOFTWARE\Electronic
Arts\EA Games\Command and Conquer Generals Zero Hour\ergc
and change just 1 number in the serial.