Open Spotify application on the Mac and select one of your speakers as shown on the image below.
Open settings (small gear icon on the right bottom) and enter your HEOS account credentials. Application must be restarted after that action.
Click GROUP icon next to the player and select another speaker from the drop down list.
Yes, Remote for HEOS requires incoming connections to automatically discover your HEOS speakers and to receive updates when their state changes (like volume, playback, source).
Played song, volume, mute, etc. has changed but the related change is not visible in the app? Sometimes it happens and the best way to fix that is to restart affected speakers, either with Reboot speakers option or by unplugging the speaker from the power source for a few seconds.