So we needed to do VPN on remote location over 3G, and i've decided to try to pair RB411 with Sierra Wireless MC7710.

It is in supported hardware list and it was detected as an LTE interface by Mikrotik right away. Great start, it got connected to the network, I'm successfully pinging around the internet.

But then then I have unveiled the problem, when i was trying to speed test my connection. Mikrotik just reboots itself under load. As soon as I start speedtest.net's test, the RouterBoard crashes and reboots itself. Any advice? I've tried underclocking, it doesn't help, and problem seems to be not about the CPU, since it barely gets any load.

this is a long shot but did you give it a try with a powered usb hub ? might be related to the power draw of the lte dongle.

MC7710 is miniPCI-e, not USB .

what RouterOS version you are using?