Installing the extension kit on a Vaillant EcoTech Plus 835

Hi, I recently received instructions through from Tado for connecting my extension kit to a Vaillant ecotec plus 835. There was no pre-existing thermostat to re-connect into so it was all from scratch.
It all looked pretty straightforward but having now connected the black and grey wires only (as directed) to the 240v RT port the heating turns on but not off.

Having looked through other discussions on other Vaillant models I note there is mention of the eBUS port and linking/bridging the 24v RT port. But this was not mentioned in the instructions.

Do I have to go through further set-up steps with the software or is the issue with the wiring?

Any help is much appreciated.