Sinfar is running an extended version of the legacy NWN1.69 (Diamond) game server which support both the NWN 1.69 client (Windows & Linux) and the NWN:EE 8181 client (Windows). You choose:
Using NWN Diamond (Recommended)
Run sinfarx.exe, It should start you game. You can pass the same parameters to it that you can pass to nwmain.exe.
Then follow those steps to connect to Sinfar Server: