As stated above.

We had a hard rain.
Bad sewage smell upstairs bathroom
We don't use the upstairs shower because water gets behind the tile.
I also know that the shower gurgles when u flush the toilet.
House was built in the 50s

Because you don't use the shower, the trap has dried out and sewer gas comes up. Pour water in the trap and then some mineral oil to keep the water from evaporating.