How to add wireless temp sensor to hot water control

niall1389

I had a working system for a few years then my wireless temperature sensor seemed to die. So I replaced it with a new one. I can't figure out how to add it to the hot water control. It now only shows the main box where it also displayed my previous wireless temp sensor which I've now removed from my setup. Hot water had still been working fine until this evening. Now even the test on the main box won't work. Please help