Backup WLAN Won't Configure After Disconnect

[over_config]

I have a pfSense box configured the use a Verizon U620L USB modem as a backup WLAN.  I've properly configured the modem to go into Ethernet over USB mode using usb_modeswitch, setting up an interface group for failover, and everything seems to be working fine.  If I kill my main WLAN connection, things properly fail over to the modem.  But . . .

After a few hours the modem randomly disconnects and then immediately reconnects (apparently, at the USB level), with this in the logs (in reverse chron) order:

Feb 27 19:48:25 php-fpm[2019]: /rc.filter_configure_sync: MONITOR: WAN2_DHCP is down, omitting from routing group WAN_GW_GROUP
Feb 27 19:48:25 php-fpm[2019]: /rc.dyndns.update: MONITOR: WAN2_DHCP is down, omitting from routing group WAN_GW_GROUP
Feb 27 19:48:23 check_reload_status: Reloading filter
Feb 27 19:48:23 check_reload_status: Restarting OpenVPN tunnels/interfaces
Feb 27 19:48:23 check_reload_status: Restarting ipsec tunnels
Feb 27 19:48:23 check_reload_status: updating dyndns WAN2_DHCP
Feb 27 19:47:59 kernel: uhid0: <hid interface="">on usbus1
Feb 27 19:47:59 kernel: ue0: Ethernet address: 00:15:ff:xx:xx:xx
Feb 27 19:47:59 kernel: ue0: <usb ethernet="">on cdce0
Feb 27 19:47:59 kernel: cdce0: <cdc ethernet="" control="" model="" ecm="">on usbus1
Feb 27 19:47:59 kernel: ugen1.3: <novatel wireless="">at usbus1
Feb 27 19:47:58 kernel: uhid0: at uhub2, port 2, addr 3 (disconnected)
Feb 27 19:47:58 kernel: cdce0: at uhub2, port 2, addr 3 (disconnected)
Feb 27 19:47:58 kernel: ugen1.3: <novatel wireless="">at usbus1 (disconnected)

Fine, no big deal.  But the problem is when the interface comes back up it does not automatically re-acquire a new dhcp address.  So when I go to Status->Interfaces I just see this:

WAN2 interface (opt1, ue0)
Status down
DHCP down    Renew
MAC address 00:15:ff:xx:xx:xx

If I click renew it will renew fine and get an IP address.  But it won't do that automatically.  I've played with the DCHP settings in the Interface page, but that doesn't seem to help.  They are currently set as follows (FreeBSD defaults):

Timeout: 60
Retry: 300
Select Timeout: 0
Reboot: 10
Backoff Cutoff: 120
Initial Interval: 10

But shorter retry times don't seem to matter; it just won't pick up a new address automatically.

Any thoughts on (a) why this is happening or (b) how I can get it to automatically pickup a new address when the modem disconnects/reconnects?

Happy to post any more logs/info that would be helpful.  Much appreciated.</novatel></novatel></cdc></usb></hid>