Friend can't join his or my server

You mean switch the server to vanilla and see if my friend can join?

Alright, I'll see if anything changes.

Vanilla works perfectly fine, and we don't need to use the dyn IP.

I think it is, but more specifically his mod. Mine seems to function perfectly fine with the server, no matter who hosts it.
I can join the both of our servers when he can't. He does have the right version, though. Are there any other variations between the modpack and the server that could cause this?