Sounds like the garage door is metal and is shielding the rf from the transmitter, is the whole building metal?, if not try moving it around in the building and see if that will work.
There probably isnt a power limiter.
You might be able to improve the transmitting aerial, but you'd need to know the exact frequency to get the length right, does it say on the unit what the frequency is.
A better way if you can might be to put the repeater closer to the building under a weathercover with a cable leading back to the supply.