Official servers have been shut down, and the pre join/create match menu has been updated accordingly. Now, to select the "internet" option and play online, you'll need to edit "region.txt" in your game folder (or MMXOD documents folder if you are using that) and set the ip address to that of a self-hosted server. Otherwise the menu will not let you advance. See the decom guide on how to self-host a server. Then restart the game for it to pick up the change. The region name can be anything but must be set. For quick reference here is the format of region.txt that the game accepts: