SmartThings Wi-Fi sub hubs disconnecting

Been using a v3 Wi-Fi hub as the router connected to a Verizon modem. Then I’ve got three sub hubs and a whole bunch of smart devices.

About 2-3 weeks ago the subs would become disconnected all at once and then reconnect. I called Samsung and they had me power cycle and then factory reset two of them.

One of them eventually got reconnected to the main while the other is gone from my plume network.

Anyone have any ideas why I was having the original disconnecting problem or how to get the lost hub back in. It won’t go into red/green blinking mode.