The Zigbee Repeaters online/offline status was a bit difficult to track. These devices typically don't need a driver - they operate when simply paired to HE as a 'device'. The HE inbuilt driver "Ikea TRADFRI Signal Repeater" does not have any polling implemented and does not send any events to the hub, thus the 'Last Activity' and the HE 'Status' properties are not handled properly and HE reported the repeater as 'INACTIVE'.
This is the first of the series of the popular Markus's drivers where the misused "Presence Sensor" capability is now replaced by the "Health Check" capability and the "healthStatus" property.
The "Zigbee - Generic Repeater (w/ healthStatus)"driver can be downloaded from this URL :
https://raw.githubusercontent.com/kkossev/hubitat-Markus-Li-fork/release/drivers/expanded/zigbee-generic-repeater-expanded.groovy
Now the ZIgbee repeaters devices online/offline stratus can be tracked using either by the healthStatus or by the 'LastActivityAt' properties :
