Gen1 door contact sensor

I am having HUGE issue with a door contact sensor.It will not connect to my network, every time it immediately flags up in the app that it has connected to the wrong network.I have deleted the sensor using the app, followed the instructions to the letter, it makes no difference.I have tried the reset option, remove battery, press tamper button while replacing battery. The led flashes once and that is it. I have held the tamper button in for a full two minutes but it makes no difference.It is a new system, everything else in the kit installed without issue but this door sensor will not do anything,.Is it faulty ? Am i missing something in the process of resetting ?By the way the instructions say to delete the item from within the app by going into the base station section and choosing the item from there. This cannot be done as the items are not listed in that section, it has to be done from the contact sensor section itself.Any help is much appreciated as i'm tearing my hair out with this stupid item.