Nighthawk R8000 used ONLY to share USB hard drive

Sorry if this has been covered, I have been searching for hours before posting this message.

I switched my home network over to use Xfinity Xfi, but because my Xfinity modem does not have a USB port I am unable to share my external USB hard drive.

I would like to shut off the wfi radios in my R8000 router and hook it up via ethernet to my xfinity modem and use it only to share my external hard drive.  I have researched and tried bridge mode, access point, etc. but cannot see the hard drive when hooked to my Xfi wireless network.

I know that the USB hard drive sharing function is working because I can access it when I temporarily turn on wifi and connect to the R8000 router.

Is it possible to use the R8000 router only to share my USB hard drive via ethernet?

If not, has anyone found a solution?  I would be happy to buy something to share my hard drive via ethernet.

Thank you!