The router doesn't advertise itself in the network as a SMB share ( as other Windows clients or standalone NASes do in network Neighborhood ) but in Explorer if you input the direct path \\<router_IP> or \\<router_IP>\<USB Device Name> you should get a prompt to input the credentials to browse the contents of the drive (the USB device's name should be visible in the router's GUI when configuring to which folder give a user access to ) and after that be able to browse the contents of the drive
There might be an issue with reconnecting to the USB drive ( 1TB Hitachi HDD , USB port powered in my case ) if the device to which the share was mapped has gone to sleep / was restarted because at least in my case with the SMB share mapped to my NAS ( QNAP , obviously Linux based NAS appliance ) it's still accessible after more than 1 week since I started testing it ; the NAS was on all this time obviously . Keeping alive the connection maybe prevents the router's SMB daemon to stall ; the share has also been accessible ( by manually imputing the path in the explorer address bar ) all this time from my Windows 10 clients when I tried to browse it's contents for testing purposes .