When the UH8-RF is installed in a metal cabinet, you should use the external aerial available here.
The aerial should be positioned outside of the cabinet to ensure the UH8-RF can function correctly.
Tip
It is worth making a note of the P2 settings on each of your thermostats and keep them as a list near your wiring centre. There is usually a sticker than can be filled out on top of the UH8-RF wiring centre. It is just in case the thermostat ever loses settings.