Strongly suggest adding a much longer USB extension cable to relocate the Zigbee Coordinator radio dongle (for less interference and better reception) as well as also adding many more Zigbee Router devices to strengthen your Zigbee network mesh. Highly recommend reading and try following all the best practice tips here:
Your current placement could not be much worse, so you need to relocate the dongle regardless. Also note that output power does not matter if the Zigbee Coordinator can not receieve incomming signals. Reception is more important, so shouting louder does make it a better listener.
That is not a good idea, best is to have all Zigbee devices on a single Zigbee network, and while you could create seperate Zigbee networks there is no way to join them at the Zigbee level.