Have you tried resetting the heatlink? That’s the brains of the whole
system and the bit that actually connects to the wifi. If you have done
that, have you tried resetting the thermostat completely to factory
settings and starting from scratch as if...
Has it connected to wifi before? If so try resetting the thermostat and
heat link box and see if it remembers the wifi. If not you may need to
pair the heat link to wifi again