That plastic part was designed to be fastened to the front of the valve and be a "throw away" after the tile was cut around it, which the tile person did not do. The first thing to do is cut the plastic out of the wall. Then maybe you can access the screws with an offset or stubby screwdriver. You might also be able to open the hole in the tile to the size of the plastic ring for better access. In other words, you got a "blue light special" installation, cheap but not correct by people, i.e. both the plumber and tile person, who did not know what they were doing Once you remove the plastic, you might find you have "integral stops" on the water lines.