I have a pc connected to a coordinator, one router and up to 20 endpoints. Some of the endpoints were accidentally set up as routers using Xctu. I do not seem to be able to reset these as endpoints via the wireless connection. I can not connect the endpoints (now routers) to the PC as I only have the wireless connection available.
You only have the ability to set as an End device if the node connected to the PC is in API mode and the firmware version installed on your Routers supports the SM command. Your only other option again is to use API mode on the PC node and perform an over the air firmware upgrade. Just understand that with 20 end nodes, you will have to have two always on parent nodes.