We're trying to play together, but none of us are showing up on the server lists. At one point, my friend playing on PC even got an "EOS server" error; what should we do?
You don't provide much information. All anyone could do is guess.
For crossplay to work, the host needs to be hosting a game that does not use mods. It needs to use only settings that are compatible with console (some settings are PC only). It needs to be a map size that is allowed on console. Also take note of the following:
To add to what Riamus said, I assume you're attempting to find a peer game so make sure you're searching for their username in the peer2peer tab and that you've selected the correct region. Double check that your clocks are synced to internet time too.