X300B

LTE-4G ref ​here

Step 1: Install USB packages

kmod-usb-net-cdc-ether 
kmod-usb-net-qmi-wwan 
kmod-usb-net-rndis 
kmod-usb-serial 
kmod-usb-serial-option
kmod-usb-serial-wwan 
kmod-usb-wdm 
usb-modeswitch

Install LTE-4G packages

luci-proto-qmi
uqmi 

Step 2: Create the QMI interface Go to Network => Interfaces => click "Add new interface" You can input any name you like. In this example, I named my interface "4G". In the protocol field choose "QMI". If you don't see QMI on the list, you have not installed the right packages or haven't rebooted the router.

Step 3: Please note that two interfaces showed up. One called "4G" and another one "Virtual dynamic interface (DHCP client)" called "4G_4". The "_4" means that the interface has an IPv4 IP address. This is based on the "PDP Type" selected in the previous step. If your carrier supports IPv6, you can use that as well (not recommended).
Get signal info status

root@OpenWrt:~# uqmi -d /dev/cdc-wdm0 --get-signal-info
{
        "type": "lte",
        "rssi": -68,
        "rsrq": -14,
        "rsrp": -100,
        "snr": 2.000000
}
root@OpenWrt:~# uqmi -d /dev/cdc-wdm0 --get-data-status
"connected"
root@OpenWrt:~# uqmi -d /dev/cdc-wdm0 --get-current-settings
{
        "pdp-type": "ipv4",
        "ip-family": "ipv4",
        "mtu": 1500,
        "ipv4": {
                "ip": "172.30.0.9",
                "gateway": "172.30.0.10",
                "subnet": "255.255.255.252"
        },
        "ipv6": {

        },
        "domain-names": {

        }
}
root@OpenWrt:~#