This is most likely a network issue. Connect only 1 CDJ to the laptop and see if the link is stable. If the link fails, try a different cable. If it's stable, then the hub / switch is probably the issue.
I had a network switch that gave me problems years ago. The switch kept disconnecting and reconnecting to my laptop for some unknown reason (even without Rekordbox running). When it would disconnect, the link would drop out in Rekordbox. In the end, I had to buy a newer switch to get everything to stay reliable.
If this is a new problem (like it was stable in the past but now it's not) it has to be either the network hardware has changed or there has been a driver update or something to that effect.
I use Rekordbox version 5.8.5. and the link is stable with 3 CDJ's connected to a Dell laptop (Windows 10).