To play Howling Peaks or Flames of the Nether , the base game needs to understand those new areas exist. That knowledge comes from (often called "Patches" or "UPD").
After installing base, update v1.18.0, and the six DLC NSPs, Alex returned to the camp. The piggy bank was happy. The merchant had new gear. And the map table showed a constellation of six new islands.
Online multiplayer requires all players to be on the same version. The Ultimate DLC Bundle: Content Breakdown