I just received my GL-M2 today and spent many hours trying to get it working with my Flint router.
As it turns out, contrary to the GL-M2 docs, 4.2.1 does not contain the modem drivers/software/etc. I had to install the snapshot 4.3.2 to get it working.
Also, there seems to be a bug in the no-ip.com DDNS scripts that have been fixed in OpenWRT, but are not getting into the 4.x GL builds. I had zero issues with no-ip/DDNS on 3.216, but it does not work at all on 4.x on either of the two routers I have tried. I believe the issue is with the url that is in the no-ip scripts; it is noip.com and should be no-ip.com, according to their documentation.