The YX500 is a great product and we get great feedback on it. Have you tried to relocate it? Usually placing a YX500-PCS or YX500-CEL in a central location resolves most issues.
Unfortunately is it not possible to split or add a combiner to the repeater. Some customers purchase multiple units & place them in strategic locations to insure coverage throughout.
Keep in mind the weaker the outdoor signal = less coverage area inside.
The inside antenna will work for you, however its directional and will only benefit when it is pointed in one direction.