Thanks. The interesting thing here is that I can remote into the network using the IP reported by the cellular modem on its internal status page. But, on failover, the IP being reported by the Dynamic DNS service and "What's my IP" is different than the IP being reported by the cellular modem. On failover, the IP address does change from my WAN address to another IP, this new IP address does get reported by the Dynamic DNS service, its just not the same IP as the modem displays on its internal status page.
In other words, if the cellular modem was being reported to my Dynamic DNS service then I'd be able to remote into the network on failover. But, as its now stands, I have no way (that I know of) of determining the actual IP address of the cellular modem on failover. So, there something odd going on with how the IP is being determined.