Posted on Asus RT-N66U FAQ
Should be included with 18.104.22.168.268 firmware (not yet available)
maybe on v22.214.171.124.321
Using 3G/4G Tethering allows sharing the Internet connection of Internet-connected mobile phone with other devices connect to the router. Insert the phone with router through USB cable, connection of other devices connected to the router with the phone can be done over wired LAN/wireless LAN (Wi-Fi). The combination of Internet-connected mobile phone and the router thus act as a portable wireless access point for devices connected to it. Noticed that if you connect both WAN and the mobile phone, the default is using WAN to Internet while failover is the mobile phone.
Step1. Mobile phone settings (Android)
Before continuing, you will need to set up your mobile phone;
1. Setting up USB PC connection as Media device (MTP) mode.
2. Enable USB Tethering: Settings→More settings→Tethering and portable hotspot→Enable USB tethering
Step2. Router settings
Connect the Android mobile to the router; you will need to set up:
A. Connect LAN to your computer.
B. Update the firmware to version 126.96.36.199.268 or later
C. Connect mobile device and router via USB port. When use 4G/LTE, using the USB extension cable to have at least 50 cm distance between the router and the mobile for best performance.
D. Now using your Android mobile phone as 3G/4G USB modem is working. Launch the setting page http://192.168.1.1, Network Map status is changed.