The fact the router supports USB devices doesn't mean it supports ALL USB devices. After all, it would need to have the necessary drivers. Fortunately, USB external storage is generic enough that it's relatively easy to support, but printers is more complicated. So it doesn't surprise me if printers aren't always supported.
There's nothing you can do except find another router that does support USB printers. I happen to have several ASUS routers that support tomato (third party firmware) and which I configure as client bridges to wirelessly enable my USB-only printers. So that's another avenue as well.
There may even be USB print servers still available. But since the dramatic price drops in network printers over the past few years (wired and wireless), they’re much less common, esp. when it comes to compatibility w/ newer printers. Even so, they often (like even my ASUS router) can’t support all-in-one features, only basic printing.
Truth is, unless you’re hanging on to a favorite “relic” for some good reason (some pp just can’t give up that old and trusty 2005 USB printer, or have a closet full of ink/toner), you’re usually far better off (at least in terms of cost) just getting a new networked printer. The real cost in printers these days is not the device anyway, it’s the consumables.