Tested with 6.29.1 on RB2011 and NetMetal5
Set ppp interface to
info channel: 0
data channel: 0
profile: default
clear out user, pin, apn, etc
phone: 99**3#
uncheck dial-on-demand
Comes up, then drops for a second, then comes up for good.
RB2011 note: The USB OTG port doesn’t provide enough power for the modem so you’ll need a powered hub or a USB OTG “Y” cable that can provide external power.
NetMetal5 note: Neither modem will fit into the NetMetal5’s lower cover (nor would you want it there) but a short USB extension cable will bring it outside to a small project box or other watertight box. Or just dip the assembly in PlastiDip. 
Both modems need mini-SIMs. For Verizon, it’s DIRECTSIM4G-D
Both modems have external antenna ports.
The Novatel is now selling for $14US on Amazon.
Just a followup…: If the interface if flapping a lot it’s probably because your’re sending packets out of the interface with an ip address other than the one assigned via ppp. You MUST have a srcnat/masquerade rule in place to insure that only the assigned address is used.