This is a 4MB space tp-link router, here are the steps
opkg install kmod-usb-serial kmod-usb2 luci-proto-3g kmod-usb-serial-option usb-modeswitch kmod-usb2 kmod-usb-serial-wwan kmod-usb-serial ppp comgt sdparm optional opkg install usbutils
Now, reboot the router, then go to the routers LUCI interfaces page, then add an interface
From the drop down, chose UMTS/GPRS
On the next page, you should see one of the devices names /dev/ttyUSB0, in my case there are 2 since the device provides a storage controller, the modem was on USB0 not USB1, your might be different (try both if you have 2)
Now i chose UMTS only so that the modem will always use 3G because edge is good for nothing
Then your APN and username/password as you would on a phone or anywhere else… then save and apply