This is mere speculation but I wonder if the usually included directive of KeepAlive 25 seconds has something to do w/ that. Activity on the link/tunnel could be enough to keep the USB modem from entering a ‘low/powerdown’ scenario.
I’d be temped to do some packet capturing for some period of time then see what WireShark can give you for an analysis. You’d have to determine the name of the interface you’re targeting (ip a should help). At least you might get an idea of what’s happening to the traffic &/or see if there’s a periodic timeout causing the issue. IDK; again, speculation.
I really want to have it as stable without a vpn though, for several reasons (one being… The cost of the vpn).
Well, worst case scenario: get another VPN provider for ‘day to day’ use… I can tell you personally Surfshark via WG works just fine w/ f/w 4.2.1-r4. They’re dirt cheap (< 2.00USD/mo) if inquire about a long term subscription (sometimes not openly advertised).