So I have a working USB 320 U (Took a while but now running)
I have Netgear Nighthawk R7000 running stock firmware
Natively, it does not support USB modem (Has USB ports)
However, it appears that DD-WRT supports USB modems, inlcuding my model - https://www.dd-wrt.com/wiki/index.php/3G_/_3.5G (Scroll down to Sierra). Seems the prominent release is Kongs.
Will this work?
Tomato or DD-WRT?
So far I have stuck with stock firmware to get hardware acceleration (whatever that really means in practice)
Possibly
Try it and see
At worst you can reflash stock firmware back onto it
Also as a fyi you are better off posting this question at the ddrwrt forums as you'll get much better answers