I'm having the exact same issue. Our static IP on the primary WAN port is fine, the optional port is into a backup (Comcast) line pulling DHCP. I had the WAN Failover working for about 2 days, then it stopped working all of a sudden.
Anyone else having this issue? I had a complete failure of the router a week ago on the WAN ports. I would hope we can get an answer to this as it's an advertised feature of the SA 540 that doesn't seem to be working well.