Elkatherm heater pairing issue

[Bob529]

Hi, These electric heaters have been installed and are generally operated via the HeatApp on a mobile phone. However the pairing has been lost and I am having difficulty re pairing. The installer mentioned that I need to add a module to each zone and to pair, press the temperature button down to zero degrees and then attempt to pair. Do that on each radiator. I've attempted several times but cannot add/allocate a heater to a zone. Any advice please?