Use separate, correctly rated connections for power and the floor sensor.
If a factory cold lead cannot reach the thermostat, a qualified electrician can route it to an accessible junction box and continue to the thermostat with wiring and connectors rated for the circuit, location and applicable code. Keep the equipment-grounding path continuous. Do not cut or extend the heating portion of the cable, or bury an ordinary wiring connection under the floor.
The floor sensor is a separate low-voltage circuit. Keep it segregated from line-voltage wiring and follow the exact thermostat’s permitted extension wire, total length and connection method. There is no universal sensor-extension length for every thermostat: OJ’s general guidance explicitly says the precise instructions may specify a shorter maximum.
Place the sensor between heating runs as its manual requires. Isolate power before work, then record heater/sensor resistance and verify the thermostat and ground-fault protection before use.
Sources: TempZone cable installation manual (power-lead routing and floor sensor), and OJ sensor-cable-length guidance.