The opposite direction is described in usb.reverse.tethering

USB tethering is used to connect your OpenWrt-Router over your Smartphone with the Internet. Follow:

• https://forum.openwrt.org/viewtopic.php?pid=173399#p173399
• if not clear enough, please read usb.essentials in this wiki

This can also be completly done by using the LuCI web UI.

Install USB Basic Support if not already installed by default on your OpenWrt router.

After you finished with USB Basic Support you have to install some Kernel packages to get USB tethering support.

opkg update
opkg install kmod-usb-net kmod-usb-net-rndis kmod-usb-net-cdc-ether usbutils udev

usbutils and udev packages are required to reconnect the phone correctly to the router.

Enable USB Tethering on your Smartphone and connect it with the USB cable to your routers USB port.

Use the new usb0 network device as WAN connection and set the protocol to DHCP client mode.

uci del network.wan
uci set network.wan=interface
uci set network.wan.ifname=usb0
uci set network.wan.proto=dhcp
uci commit network

After commiting the changes to the network configuration bring up the WAN interface to obtain an IP, gateway and DNS address from your Smartphones DHCP server.

ifup wan

Login to the OpenWrt router with SSH at 192.168.1.1.

logread

ifconfig $(uci get network.wan.ifname)

ping -c 4 www.google.com

Have fun