Draytek Vigor 2926AC Port Redirection lost when Failover activates

[JohnDohe]

I have a DrayTek 2926AC router that has an Ethernet feed as it's primary WAN and a 4G router (factory default settings + APN) as the Failover WAN.

I also have Port Redirection set up in the DrayTek 2926AC.

When I disable the primary WAN, the failover WAN activates and the internet is restored HOWEVER the Port Redirection fails.

When I restore the primary WAN, the Port Redirection also comes back.

Anyone know if this is a design flaw?

Or does someone have a workaround?

Thanks!