When in doubt, add more Zigbee Router devices 
When it comes to Zigbee networking mesh it is almost always best to just have many “good” Zigbee Router devices than only a few “great” Zigbee Router devices, but still, having a few “known great” dedicated Zigbee Router devices placed in strategic location can be a very good idea, and that is something that I sometimes recommend to new Zigbee users that plan to start with a few devices before they add more, and if they do not get will not get enough coverage or range needed for their entire home.
There are also some specific Zigbee Router product tips about that here → Zigbee networks: how to guide for avoiding interference + optimize using Zigbee Router devices (repeaters/extenders) to get a stable mesh network with best possible range and coverage
Personally I can stongly recommend convert a few of ITead’s “Sonoff Zigbee 3.0 USB Dongle Plus V2” (model “ZBDongle-E” based on Silicon Labs EFR32MG21) into dedicated Zigbee signal repeater / Zigbee range extenders by flashing them with Zigbee Router firmware and powering them with a simply USB-chargers. While maybe not the prettiest solution to look at, if you make sure they are permanently powered then joining/pairing three or more to your Zigbee network and spreading them around in your home will create an extremely stable backbone in your Zigbee network.
Action: Buy a few known good dedicated Zigbee Router products and place them strategically as Zigbee repeaters. Personally, I suggest buying and adding at least three such devices.
Tips
- Examples of suitable producets: the “IKEA Tradfri Signal Repeater ” and “Aeotec Range Extender Zi ” are products that work very well out-of-the-box, while a more powerful alternative is to make yourself some semi-DIY variant by flashing the correct Zigbee Router firmware to Zigbee Coordinator USB dongles and then using them as stand-alone Zigbee Signal Repeater devices in USB-chargers for power, like, for example, either the Sonoff ZBDongle-E (EFR32MG21 based) or the Sonoff ZBDongle-P (CC2652P) work great as dedicated stand-alone Zigbee Signal Repeaters with recommended firmware.
- Search community forums for more “Zigbee signal repeater” or “Zigbee range extender” Zigbee Router tips
- Buy a few additional new Zigbee USB adapters to use after re-flashing them with Zigbee Router firmware
That is actually not entirely true. All types and models of Zigbee Router devices are definitely not created equal. How they perfom will depends on exactly what antenna type and design, Zigbee radio chip model used, Zigbee stack revision, Zigbee firmware version and configuration.
Most commercial mains-powered Zigbee Router devices will not have a radio chip or firmware configuration that offers the same performance as a DIY Zigbee Router made from a convertered Zigbee Coordinaor dongle with external antenna. Older chips also ussually have worse performace than newer chips.
Yes it is true that there are government regulations for the maximum transmit power-level for low-powered devices (around 14+ dBM in Europe and around 20+ dBM in North America), …but that does not mean that everyone configure their to use the maximum (fact is that most devices that ship use much lower transmit power than maximum allowed), nor that everyone follows the regulations.
Regardless of avove, relocating your Zigbee Coordinator dongle will be among the most important steps.
Begin by relocating your Zigbee Coordinator dongle by using a longer (shielded) USB extension cable and make sure connect it to a USB 2.0 port (and not a USB 3.0 port) or specifically use a USB 2.0 cable. EMI/RMI/EMF interference can easily kill signal reception for the Zigbee Coordinator, which is why it is so extremely important to keep it away from all possible sources of interference and make sure that the radio frequency range of your WiFi channel(s) do not overlap/conflict with the radio frequency range of your Zigbee channel. Again, that and more is covered here → Zigbee networks: how to guide for avoiding interference + optimize using Zigbee Router devices (repeaters/extenders) to get a stable mesh network with best possible range and coverage
There are also some specific tips about that there, including how to convert an Ethernet cable to make a very long (but passive/dumb) USB extension cable by buying RJ45 conversion adapters → Zigbee networks: how to guide for avoiding interference + optimize using Zigbee Router devices (repeaters/extenders) to get a stable mesh network with best possible range and coverage
You could do that with “zha-toolkit” but understand that in your situation there is no point with a before picture, that would just be a waste of time at for you right now in your current situation. You first first need to take action to sort out all low-hanging-fruit issues before troubleshoot any deeper → Zigbee networks: how to guide for avoiding interference + optimize using Zigbee Router devices (repeaters/extenders) to get a stable mesh network with best possible range and coverage
Also understand reported RSSI and LQI values can sometimes be arbitrary and not always useful, read:
and
That is correct. There are no such things. Time to add that to docs → Mention another Zigbee limitation in ZHA docs by Hedda · Pull Request #33417 · home-assistant/home-assistant.io · GitHub
…and what do you know, that is also covered here → Zigbee networks: how to guide for avoiding interference + optimize using Zigbee Router devices (repeaters/extenders) to get a stable mesh network with best possible range and coverage
I worte that guide so that I would not have to sound like broken record that repeats itself 