Make sure that you never power off any mains-power Zigbee Router device on purpose as Zigbee Router device are meant to always be available or else devices connected via them will loose connection. This is a common mistake people do with Zigbee lightbulbs as many connect then to dumb wall-switches which is a big no-no unless you specifically buy Zigbee lightbulbs that are Zigbee End Devices (non-routers).
Other than that you probably just need to add more always-on mains-power Zigbee Router devices and place them in strategic locations to get better coverage and range as well as improved redundancy.
Personally I would suggest buying three or more Sonoff ZBDongle-E USB adapters and converting them into dedicated alway-on Zigbee repeaters/extenders by flashing them with Zigbee Router firmware.
Recommend read and take actions to try to follow all the best practice tips here: