How to Make An Online Multiplayer Game With My Own Server?

I actually have my own Server Guy, He will make the rooms and all but how should I make an real time multiplayer game from that?
I’ve searched many places and they give me SDK but I’ve to use their servers also it goes same for unity.
Thanks in advance.

Define by what you mean by your own server. are you using the Unity networking API or are you talking about Socket Programming?

You should be deciding what your general server architecture and requirements of those servers will be. After that, decide on what networking package best fits those requirements. Then follow whatever method that networking package uses for creating servers.