sdk for multiplayer games.

Hi. I have developed a turn based board game. I wanna include multiplayer functionality. I have several options and all of them have drawbacks:
1- photon : it only supports 20 concurrent users.
2-parse.com: it does not have api specifically for games.
3-shephertz: its api for xamarin seems to be outdated.
4-do the server side myself: it is a lot of work!
What other options do I have? (by the way, does anyone know any api to use in xamarin forms, so I don't have to write separately for android and ios?).
thanks.