Take a look at SmartFox Server. They have a C++ api that you can use to connect with using UE4.
The bottom line is you need to look at using some middleware for your game. Writing a whole MMO networking engine from scratch requires expert network programming skills in C++.
Take a look at SmartFox and Raknet, or Photon as others have suggested.
In all honesty, unless you have at least $1 million in budget don’t even try making an MMO.
Start off with something simple like multiplayer pong, and see how long it takes to complete a game like that.
Then multiply that development time by 1000 and you’ll have a better idea of what it takes to make a simple MMO.