We use RB 411u boards and Huawei EM770 3G modems for our projects. Our problem is about PPP does not connect to internet after 2 or 3 months until we remove the SIM card from the board and put it into our phones and connect it to internet once.
Then we put the SIM back to the board and PPP can continue to connect to internet.
It seems like a kind of sleeping situation after a while. We do not know what cell phones send to SIM operator to awake it again but 411u or Huawei modem does not do that and PPP sleeps.
We must remove and activate the SIM once every 2-3 months.