SSL VPN during failover?

Just had my first failover situation. My main fibre connection went down and the failover using LTE (mobile broadband) worked. Dyn DNS was also updated after failover.

However, I couldn't connect SSL VPN. My UTM  (v9.505) is configured to listen on SSL VPN on the fiber connections external address. And it is not possible to choose an interface group for listening for SSL VPN.

What is the trick to make SSL VPN work during failover?