I wanted to share a site I recently came across with. I have been struggling to find complete and straightforward explanations/exercises for prepositions and their cases in German. In this one, you can find explanations/exercises for local, temporal, and their declension in the four German cases.
The site is much larger and covers much more than just prepositions. However, I have only been able to look into those exercises. Fell free to tell me if you found the rest of the site useful.
Hope you enjoy it!