External Storage for Wifi router with Ethernet?

I am simply trying to find out whether I could use a USB to Ethernet adapter to connect my Eero Pro (or any router with an Ethernet port) to an external device (with a USB port).

No, this would not work. You will need something like MR220AC mesh routers which comes with a USB port. You can then connect external drives and use them as a NAS.I hope this helps.