Welcome to the forum.
You may have a problem with the circuit board on the rear unit, which is part of the lower unit. If you aren't afraid of electricity, you can swap circuit boards from front unit to the rear, and see if the rear unit starts working.
Also, check the wires that connect to the thermostat. One of the wires may be broken or corroded to the rear unit. Lastly, the thermostat may be bad, though it's doubtful since the front unit runs. Can you change zones so the rear unit is zone 1?
Two wrongs don't make a right, but three rights make a left.