Read and try to following all tiopa here → Zigbee networks: how to guide for avoiding interference and optimize for getting better range + coverage
Best case you only need to add avoid interference by following the tips, middle-groubdcase you on only need to do the former as well as add a few additional ”know good” Zigbee Router devices, and worse case you have one or more rouge devices with bad firmware that should be removed if causing problems (in which case you need to troubleshoot to find those, but that is the last route so start with the other things that are recommend first, avoid interference and add more Zigbee Router devices that are good).