I’m very confused by what you’re saying here, but Server functions are not removed from shipping builds… that wouldn’t make any sense. Using replication to drive an MMO is generally ill-advised but not impossible, you probably want some kind of deterministic lockstep simulation - and Unreal is neither of those things.
If you want to look into custom networking solutions, look at Raknet or something. Or you can directly send data using UE’s sockets.
Hard to suggest ideas without more information on the game. MMO can apply to a wide variety of games.