sorry for delay. seems something wrong with notifications on itch.
1. Most players amount available in European evening. In some hours most of the servers may be empty
2. Look for servers list in Multiplayer->Servers, there must be plenty of servers
3. Dont forget to sort servers by players number(right column), in rexuiz its done by default, but anyway try to click it.
4. The game really isnt connected with itch app. Try to use launcher directly to start the game.