If MESH blocks have been disconnected from MESH Hub, please check the following:
１．Is your MESH Block still turned ON?
To check if your MESH Block is turned on, click the icon on the bottom half of the MESH Block. If the status LED turns on, that is OK. If not, then the power is off and you need to turn on the MESH Block. To turn on your MESH Block, press and hold the icon on the bottom half of the MESH Block for two seconds. If you cannot turn on the MESH Block, then you may need to charge it with any micro-USB cable.
When the MESH Block is on, the status LED turns ON by clicking the icon on the bottom half of the MESH Block.
２．Check the wireless connection between the MESH Block and the MESH Hub.
A MESH Block can disconnect from your MESH Hub when the MESH Block is too far way from the MESH Hub. MESH Blocks use Bluetooth to pair with MESH Hub, so the MESH Blocks and the MESH Hub need to always be within 30 feet of each other. Also keep in mind that physical obstacles (e.g., walls) and nearby wireless devices can also interfere with the Bluetooth connection. Try moving the MESH Block and MESH app within 30 feet of one another with a clear, unobstructed path.
３．The Status of connection between MESH block and MESH hub are enable, but frequently lost (unstable) ?
Even if you use Raspberry Pi OS (previously called Raspbian) Stretch, but the OS kernel version is latest, the problem about connection may be occurred.
In the case, try to downgrade kernel.