There are a number of articles about Zigbee in the Community Cookbook Index.
The Basics:
Zigbee relies on a mesh network of devices to allow it to use low power signals.
All devices in a Zigbee mesh fall into one of three roles:
- Coordinator:
- There can be only one Coordinator for a given mesh.
- Router:
- Routers are the backbone of the mesh and relay messages between the coordinator and end devices.
- Routing requires power so routers are mains-powered devices; they can be standalone devices but many smart bulbs or smart switches act as routers. Standalone routers are usually able to route messages for significantly more devices than those that also have another job.
- For a reliable mesh, you should plan for an absolute minimum of one router in each room and one in each hallway. Most situations will require more than that.
- End Device:
- Most stand-alone, battery-powered sensors are end devices that do not route messages from other nodes in the mesh.
Unless you changed their firmware, both your “gateway” and “dongle” are probably programmed to be coordinators. You cannot have more than one coordinator in any given Zigbee mesh.
Probably, but Sonoff sells multiple Zigbee devices that are described as “dongles”, so you will need to clarify which device you have. I’ve been using the Sonoff ZBDongle-E as my coordinator for a little over a year and it works fine. The coordinator is located in the basement and I have reliable connections to devices on the ground floor, 2nd floor, and in the attic.
Also, it is usually advisable to use a USB extension cord to avoid radio frequency interference created by the server, this is especially true if your server has USB3 ports.
Probably… routers are crucial for a reliable Zigbee mesh.