This post addresses your question.
Without the gimbal you can mount the RE210P in a corner at 45 degrees by drilling/screwing through the angled corners on the back of the cover. If you want to mount it at another angle or tilt it up/down then you need the gimbal but the most common corner mounts is just a straight 45 degree angle. Carefully pop the circuit board out of the back cover before you drill to avoid accidentally damaging it. You can pop it back in after the back cover is mounted.