The problem is in the DTS file for this and a bunch of other Ath79 routers.
These routers use a GPIO pin to control the power to the USB port and, in the past, this was defined as a GPIO in the definition, set to high by default
In 21.02 this has been turned into a "reg_usb_vbus" entry and someone forgot to add the GPIO pin definition to it. A quick look at some TPLink routers I know have GPIO controlled USB power shows the same thing, no GPIO is defined.
The whole reg_usb_vbus entry seems to be a problem area anyway.
I have come across a number of routers with this in their DTS file and, even when the GPIO is defined, don't turn on power to the USB port. I've had to change back to the old method to get it to work. The Netgear R6220 is one just had to fix today.