Zigbee devices that won’t connect sometimes need to be reset to factory defaults. Some do this when pairing, some do not. So check the device manual if there is such a procedure.
If that does not work, also check how crowded wifi channels are, and check that with the channel used for Zigbee. Maybe a neighbor bought a new router, maybe yours switched channels, … Carefull, the channel numbers cannot be compared directly, both protocols use different numbering.