But the link is reliably dropped after a few hours. The dropping of the link may be connected to following the schedule I have set for the thermostat; I have not verified this.
This is likely the issue. There are two places to link the Temp Sensor and Thermostat for averaging. One is on the thermostat card for manual controls, then there is an option for averaging within the schedules.
Instructions on adding the temp sensors for averaging within schedules is found in the link below. The link jumps to the relevant section of the guide.