Trouble with Huawei LTE USB Modem on FortiGate 60E

I have a FortiGate 60E, running FortiOS 5.6.6, with a Huawei E397 4G-LTE connected to the USB port. I'm using an Sprint (GSM, US) SIM card. I know the modem, SIM, and GSM are good because I can plug it into my laptop and get internet connectivity through it just fine. However, I'd like to get this working on my FortiGate 60E as a backup internet connection. I've read the guide on configuring modems for FortiGates, and I've been able to get the modem to show up as a "wwan" interface, connect to Sprint's network, pull a DHCP lease from Sprint, with a DHCP-obtained default route and DNS server, and I can ping out to the internet from the FortiGate. However, the wwan interface still appears as "down" to the FortiGate, and I can't ping out to the internet from the lan, even though I have default route and a lan>wwan policy that allows all.

I can't figure out why, so any help would be greatly appreciated.

Attached are the relevant portions of the config, plus some debug output. I'm happy to provide any additional information.

Thanks,

Victor Weis