Nokia 1 phone usb tethered to Asus 51U router..insufficient charge

works fine..unlimited data on phone with a usb cable connection from micro usb in phone to normal usb 2.0 on router. These routers can use usb connection is selected as source. Problem…the half amp output of the usb 2.0 port is not sufficient to match what’s being used while in the tethering mode and after about 15 hours the battery in the phone is down to zero even though the phone is being charged. Any way around this?