Hi there and welcome to the community!
Unfortunately, the game is around 8 years old now, and it definitely shows its age in terms of community population. EU servers are usually active most of the time (minisiege, Tropical death/paradise, etc). For NA bot survival, APPL NA Zombies, and commander battle servers are usually populated.
One way though that you can help delve deeper into this game is to join a regiment, a group of players which do linebattles/sieges with each other. If you are interested in joining mine, the 15e, just add me on steam
here and i can help you get set up.