How do I connect an expansion unit/power module to the DITRA-HEAT-E-RS1 smart thermostat? What is the maximum length of signal cable between a power module and the thermostat?

Connect the thermostat AUX 1 & 2 ports to the power module AUX IN ports.

The maximum length is 80 feet (25 meters) , and with 20 AWG signal or communication wire (recommended). See the installation guide for further details.