Install a Tectonic - Spawning Pool server through the Control Panel (Servers → Select your server → Game servers → Add Game Server → Tectonic - Spawning Pool)
Got my modded server set up very quickly and easily! Server runs 24/7 without any problems and I've been playing with my friends daily.
The in-built mod manager is very nice, it allows me to switch game servers easily and switch modpacks if me and my friends want to play something different.