Welcome to the forum. Please don’t post the same question in multiple forums. Tends to clutter up the board.
The cable is adjustable by tightening up the wing nuts on the rear brake cam. Try that and if it doesn’t help you’ve probably got water in your drum which will keep the rear brakes from working well.
Only way to fix is to take it apart, clean out well, replace all the seals, and all of that will be a temporary fix if you ride in water at all.