I can't get in the server

It's been explained in minecraft itself that playing on the wire doesn't have to be wifi but it could also be a cell phone data, correct?
But yesterday I tried to get into one of those minecraft servers and I couldn't get into one when I walked like this " you must be connected to wifi to play with your friends".. Why? 

Whether now entering the server should use wifi, not with cellular data then how about those of us who don't use wi-fi please explain!