Hi my name is ***** ***** will answer your question today.
To clarify I assume the model of thermostat is a TP5E-RF (wireless) and the receiver is a RX2 or RX3?
The low battery indicator I assume is on the LCD display of the Thermostat (sender), this means the battery has less than 15 days of charge before it powers off.
Slide the bottom half of the case side ways to reveal the battery compartment, and replace the AA batteries, the issue you are facing is most likely due to the signal not reaching the receiver, replacing the batteries should resolve this issue. If it doesnt you may need to re-sync the sender and receiver, please let me know if you still have problems after replacing the battery.
Do you still require assistance or have you now been able to get the thermostat working?
If your happy for the case to be closed, please rate the service you have received.