You can try to double check all the vpn parameters if anything changed or you can try to use a different custom port in case a ISP has changed/blocked something on the network. Are there any port-forwarding/firewalls in between that might have been changed?
Working on VPN disconnection issues are difficult as it may be caused by a lot of influences.
You may want to contact our reseller (your purchase point) where you bought the units from to provide the technical assistance. Or, you can open a support ticket to allow support team check from the devices. Mention the serial numbers in the ticket and Turn On the Remote Assistance.