If you have a 3G/4G USB dongle, which is not battery operated, just a USB stick, you might want to check out the instructions here
The device I am using at the minute is the ZTE MF920U, I have also used a bunch of similar Huawei devices, they all work in exactly the same way
Start by installing a few packaged, hopefully you are familiar with loggin in with SSH, if not, you can get those packages from within the software section of the luci interface
opkg update && opkg install kmod-usb-net-rndis kmod-usb-net kmod-usb2 usb-modeswitch kmod-usb-net-cdc-ether
Once those are installed, reboot, then login to luci
Connect the 4G modem via USB, then Go to interfaces, then add a new interface, select the new eth device that appeared, give it a name, and you are all good to go