Install a NeoForge 21.0.145 (MC 1.21.0 ) server through the Control Panel (Servers → Select your server → Game servers → Add Game Server → NeoForge 21.0.145 (MC 1.21.0 ))
My server through ScalaCube was super easy and convienient to get setup, and adding in custom mods was very straight forward and simple -- Everything has been stable since it's been up!