Samsung Galaxy s7s have a defect that is denied by the manufacturer, the rear camera lens spontaneously shatters without warning. Often people will pull their phones from their pockets to find that the camera lens has shattered when it has not undergone any stress, but Samsung denies this and therefore refuses to fix it. Well, the broken glass got into my camera and broke the camera so it would no longer focus at short distances.
I ordered one camera off some company from China, and when the camera arived it did not work at all. I got my refund and gave up for a few months until i found the camera on ifixit. I had already ordered a fit on ifixit and since I had tore it apart before putting in the first camera, i just popped it in and it worked like a charm.
If you are worried about your back cover coming off and don't want to get any new adhesive, just get a phone case. If i had bought new adhesive every time I tore my phone apart I would be spending hundreds. Now i just take off the case and pry the cover off with my fingernail to access my phone. This may affect the water resistant property of your phone though, and you should be even more careful around water.