Some water heaters have 1 or 2 or 3 valves that must be turned to
switch from winter bypass to summer hot water modes.
It's up to you the owner or you and your dealer to find them and know
what to do with them.
Your LP would not fire most likely because your tank was already hot.
Dual heat water heaters typically have the LP thermostat a few degrees
cooler than the electric side.
This way you can have both modes on and the electric will keep the tank
hot until you use enough water for it to get behind and then the LP will
fire up to help.
If you want to test your LP you should find a circuit breaker in your
power center that is for the electric side of your WH.
Flip that off and run some hot water.
The LP side should fire up pretty quickly. The tank is small and it doesn't take
much to trigger a heat cycle.
Glad you figured yours out!