I resolve the problem for a Digi 3G stick with APN= internet and dialer nr.= *99# without credential for my model MSR954:
The position on the router USB0 is down and USB1 is up.
<Router> system-view
[Router] dialer-group 1 rule ip permit
[Router] apn-profile DIGI
[Router-Apn DIGI] apn static internet
[Router] controller cellular 0/0
[Router-Cellular0/0] serial-set 0
[Router-Cellular0/0] quit
[Router] interface serial 0/0:0
[Router-serial0/0:0] ip address ppp-negotiate
[Router-serial0/0:0] ppp ipcp dns admit-any
[Router-serial0/0:0]ppp ipcp dns request
[Router-serial0/0:0] dialer circular enable
[Router-serial0/0:0] dialer-group 1
[Router-serial0/0:0] dialer timer autodial 5
[Router-serial0/0:0]dialer number *99# autodial
[Router] line tty 1
[Router]user-role network-operator
[Router-line-tty1] modem enable both
Very important is to see the USB stick become active when is conected to the router. I have an Orange stick with this problem. When this is conected the router do not detect it.