I did finally get a Pantech UML290 USB to work with a 951G-2HnD with v6.0 installed.
Add a new PPP client
Edit it and switch to advanced mode
Phone #: *9***3#
Dial command: ATDT
(so if your number is 555-555-1212 it would be email@example.com
Dial on demand: yes
Add default route: yes
Use Peer DNS: yes
Don't forget to setup a srcnat masquerade for it
Only issue I've found is that the connection keeps going up and down a little to fast for my taste (hangs up once its done or even partially done loading a webpage). No amount of jiggering with the timeouts helps. So there is a ton of "dialing.. hunging up" in the logs.. (yes mikrotik spells it "hunging up".) So the end result is that the connection feels flaky... but flaky is better than offline I guess... Also I found everything seems to work a little bit better if the USB modem is plugged in at boot time. Plugging it in afterwards does work, but it feels even more unstable...
speedtest.net says I'm getting 4G speeds and it somewhat falls gracefully on the USB modem if the ether1 goes down (but not so graceful if ether1 is UP but just unresponsive). But I still see traffic from time to time try to go out the 4G modem so if you have a limited data plan its probably not a good idea to let the ppp interface enabled.
You'll probably have to come up with a script for the script scheduler to check for internet on your primary interface and if none, then bring up the ppp. (If this is even possible. I've learned if you've never done it, its better to assume its NOT possible first... with routeros anyway...)