Hello all.
Wonderful, thank you so much: qubes-tunnel.service seems to work now – I just waited and did all updates (stable releases), as always.
Specs: Qubes 4.0; tunnelvpnVM based on fresh downloaded Debian-9 template prepared for the challenge; standard Linux .conf from AirVPN, UDP-443; tunnelVM directly connected to sys-net (Debian 10), without sys-firewallVM (Fedora 28).
If you need any logs, prefs, further specs, etc., just ask.
Best regards,
p b