Cannot Join Custom Multiplayer Game

Hello. I’ve played Grim Dawn for years and love it. I finally got a friend to play it with me, and we decided to try a custom multiplayer game. We had both downloaded a few popular mods, such as Dawn of Mastery’s, Grimamillion, and Reign of Terror. All these mods work for us as single player. But when either of us try to join the other’s multiplayer LAN game, we get an error that says “This game requires a custom map …”. Is multiplayer supported on these mods?

Thanks in advance for the help. I couldn’t find any information about this elsewhere.