I followed instructions shared in  and successfully let the device operate. I deployed snapshot build at beginning, but sadly I ran into a bug. Obviously, stable version is the better choice. I put the mir3g-v2 description file back to the stable branch and built with luci web interface. The built firmware could be downloaded here.
Update: There’s an open-source tool  to get the root shell from the device without hijacking the chips. Please leave a comment if you need video guideline.