Hexen 2 pretty much only has one choice for source ports these days, at least one that is still upkept. Hammer of Thyrion. Download that, open hwcl.exe or glhwcl (if you want fancy opengl rendering), and connect to clovr.xyz.
If the server is running the expansion pack you may have to make a shortcut and run it with the -portals command. Currently no coop since my network hates quakes origina netcode, so only Hexenworld games (ctf and siege atm).