Most router manufacturer’s firmware doesn’t support phone tethering. But there’s 1 exception to that rule…
And every single ASUS router that runs their awesome ASUSWRT firmware is capable of tethering to your 3g/4g phone and sharing it’s data connection (android only).
The only other firmware’s that support tethering are OpenWRT and DD-WRT. The problem? These aren’t stock (manufacturer-installed) firmware. You have to ‘root’ your router and flash the firmware yourself, which comes with the risk of ‘bricking’ your router (breaking the software so it won’t turn on or work properly).