Hollyland Solidcom C1 Pro User Manual page 14

Product Usage
After the upgrade is completed, a pop-up window appears on the hub,
prompting you to assign a number to the pairing headset. Numbers in
yellow are in use and hence not selectable, while numbers in gray are
selectable. Select an appropriate number and choose Confirm to complete
the pairing process.
Start you work.
Note:
When two hubs are cascaded for roaming, only one headset can be paired at a time.
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