Hardware: RB751U
ROS: 5.11
USB Modem: Verizon Novatech USB551L (on the compatability list as of 5.9)

I set this up, and connected it. The RB detects the USB modem, but when I try to make a PPP connection, I get this in the log:

initializing modem.
resetting link
resetting link, could not acquire serial port.
dialing out.

...repeats over and over.

I tried with and without apn=vzwinternet, but cannot seem to make any headway with it.

0 name-ppp-out max-mtu=1500 max-mru=1500 mrru=disabled port=usb1 data-channel=0 info-channel=0 pin= user= password= profile=default phone= dial-command=atdt modem-init=atz null-modem=no dial-on-demand=no add-default-route=yes use-peer-dns=yes keepalive-timeout=30 allow=pap,chap,mschap1,mschap2

Same issue. Did you figure out the problem?

My solution was to buy a cradlepoint router.

Any luck anybody?