I think that I solved the problem of seamless failover.
Mikrotik router is configured for Recursive failover. Ether1 is ISP1 (cable modem), LTE1 is ISP2 (Android phone / 4G USB modem). The router runs of off a 12V battery.
Then I went to the OpenVPN website, registered for their Cloud (location) service and created two tunnels, same locale (the service is free up to 3 tunnels).
I downloaded the config files: tunnel1.ovpn, tunnel2.ovpn (choose OS: Windows).
I installed the OpenVPN client in Windows on my laptop and imported tunnel1.ovpn (because Mikrotik doesn’t support UDP and TLS in OVPN client config).
Then I installed on my Android phone the OpenVPN client app from the Playstore and imported tunnel2.ovpn. I activated both VPN connections.
After that I made a Skype video call, and simulated power outage by unplugging Ether1 cable and after a few minutes I plugged it back in.
IT WORKS! Seamless failover works, Skype does not drop the connection, small stuttering was observed under one second during failover. This also should work with other video conferencing software.
EDIT: The ether1 connection and LTE1 have to have the same TTL. Set ether1 to lte1 TTL:
/ip firewall mangle add chain=postrouting action=change-ttl new-ttl=set:117 ttl=greater-than:200 out-interface=ether1