With openwrt and the rndis package and prerequisites installed, and friendly wrt, I had a good result with a usb android phone put into permanent tether mode in developer options.
I am very curious about M2 4G options that work on debian/ubuntu/wrt.
I haven't tried any usb modems as I don't have any, dd-wrt and open-wrt forums have rough guides that may be of some help.