The router does not automount the USB drive, as it should when inserted. I tried it on my router with the same result.
Instead of using the limited GL.Inet GUI, use the LUCI GUI. Before doing that, install the luci-app-samba package under APPLICATIONS → Plug-ins, if it is not already installed.
Under MORE SETTINGS, click on Advanced to go the the LUCI GUI. Under the System menu at the top, click on Mount Points to list all your mounted file systems without having to go into SSH. The USB drive should be /dev/sda1 that is missing in your screenshot.
If so, click on the Add button to manually add the mount point in the pop-up screen. Pick /dev/sda1 in the Device box and type “/mnt/USB” (or whatever you like) in the Mount point box. Click on Enabled at the top, then go to the Advanced Settings tab and type “rw” in the Mount options box. Click on Save button to return to the Mount Points screen and click again on Save & Apply at the bottom. For me, the USB drive then displayed as mounted as /mnt/USB (or whatever you entered before).
If /dev/sda1 does not show up in the Device box, the you have to go into SSH and type the command “block detect”, which will detect the USB drive and display its UUID. Instead of picking /dev/sda1 in the Device box, pick the UUID in the UUID box.
Next, under the Services menu at the top, click on Network Shares to list your shared directories. Click on the Add button to manually add the shared directory. Type “GLUSB” (or whatever you like) in the Name box and “/mnt/USB” (or whatever you entered before) in the Path box. Clear Read-only, click on Browseable, click on Allow guests and click on Save & Apply at the bottom. For me, the USB drive then was accessible as GLUSB over the network.