If you’re using MiniPCIe 3G cards on Mikrotik Routerboards, the new RB912 may be a much better choice than the older and cheaper RB411u, as the RB912 has the capability to interrupt or cycle power to the miniPCIe interface via software, which the RB411 lacks. In some particular circumstances*, such a full ‘power cycle’ of the 3G card can be necessary to restore 3G connectivity, and so for some installations, the RB912 is an essential choice when considering overall reliability.

Possibly because of this additional power cycling feature, I’ve found that 3G miniPCIe cards in the RB912 can be a bit tricker to get up and running. Out of the box, with a Sierra Wireless Mini-PCIe card (MC0705) installed, the card is inactive in the RB912 and the board will not have a 3G interface. In this case, it is necessary to first enable USB power to the MiniPCI interface by running:

/system routerboard usb set type=mini-PCIe usb-mode=force-host

If it doesn’t immediately come up (and instead tries to create an LTE interface), it’s quite possible that you need to disable DirectIP on the port first, hard reboot for good measure, then run the following:

/port firmware set ignore-directip-modem=yes

Then, hard reboot for a final time and run the ‘usb set’ command as above again.

We can also use this power-cycling feature to create a more effective interface watch-dog script, that will monitor and reset power to the card if the interface goes down:

/interface ppp-client monitor ppp-out1 once do={:if (\$status != \"connected\") do= {:log warning  \"PPP Link Down, reboot USB\"; /system routerboard usb power-reset duration=30}}

Add the script (or just that command) to the System Scheduler at any interval. Note that the board should have time to try to connect and bring that interface up before the script runs and power cycles it, so a start delay of 5-10 minutes (after boot) is a good idea.

* One common scenario here is when the network carrier has ‘cut off’ service for the SIM because the SIM’s data usage cap has been exceeded. We ran in to this often, and found that in most cases, there was no way to get that SIM back online other than by power cycling the radio itself. This seemed to be a limitation of the network/carrier.