RT-66U as Failover - Failing - Verizon JetPack 6620L

Greeting All...

Just wanted to see if anyone else has had, or is having, such an issue as the one I am outlining below.

Wanted...USB Fail Over from main WAN post when Internet drops to a Verizon 6620L Jetpack tethered.

Router: RT-66U
Firmware: 378.55_0
WAN Post: Cable Modem
USB Port 1: JetPack 6620L running in USB Tethering mode
USB 2: Sony 64 Gig Thumb Drive

Router Running with 2.4Ghz and 5Ghz radios with one 2.4 guest access SSID.

Issue 1: Failover does not occur after some time....

RT-66U as Failover - Failing - Verizon JetPack 6620L