Hi.

I have a similar configuration but I'm just curious about your settings:

Is your WiFi always active so that it can connect when configured APs are in range or you managed to turn WiFi / Cellular on and off automatically?

Also, for the normal exchange of information between applications each layer must notify the 'upper' layers / apps the problems with connectivity and the upper layers / apps react accordingly.... (this is what I can recall from the OSI standard model :-D ).

For example: if you have an active ssh session and you switch the 'underneath' transport layer from WiFi to Cellular Data or from Cellular Data to WiFi, the ssh software should (and it does) keep track of disconnection / re-connection and resume working as if nothing happened, provided the time required to reconnect is short enough....

For VPN connectivity I use SecureFishNet: it's not opensource, but I haven't had time to solve my VPN problems with Connmand
Regards.