I was once able to increase the RF range of Marantz RC5200 remote control systems using a passive antenna, using two slightly different approaches.
I found out what the operating frequency of the remote was. Zigbee has various different frequencies, so you'll have to find out the frequency for what you're using.
I built two ground plane (monopole) antennas cut to the right length for the frequency. To do this, I took a piece of RG6 and removed the jacket, shield, and dielectric from a quarter wavelength of the inner conductor. I worked out some other pieces of wire to be the ground plane.
One of these was mounted near the component that was furthest from the RF receiver. An RG6 went from there to the RF receiver, and I placed the antenna within a few inches of the RF receiver. The positioning of things basically doubled the remote range.
The other one was used simply to get a long range in the rest of the house. Since the Marantz control was unidirectional, I could add an RF amp to make the whole thing more sensitive. Your cable remote wouldn't happen to work if it didn't get feedback from the cable box, would it?
Those are the ideas that worked with that product. Maybe this can be done with Zigbee, too.
A good answer is easier with a clear question giving the make and model of everything. "The biggest problem in communication is the illusion that it has taken place." -- G. “Bernie” Shaw