I don't have enough reputation to comment hence this answer but consider this a suppliment (comment) to the query asked for explanation of /g in answer given by Phox+baruchiro :
So the default drive letter is assigned by router which is g
in case of tp link router normally. So your path to access the usb attached to router assuming router ip address is 192.168.1.1
will be : smb://192.168.1.1/g
for explorer while using cifs utilities just use //
as stated in the answer. Unless you do not change this in the router firware which needs custom firmware (assuming you use router as is from the manufacturer in this case tp-link) the drive letter cannot be changed.
Router Settings Preview for USB setup
There might be some models which might use different letter but idea remains the same and is not something you can modify.
So your root (parent) folder of the usb storage device attached will be accessible using smb://ipaddress/g
while sub directories(folders inside) can be acces as smb://ipaddress/g/foleder1
or smb://ipaddress/g/folder1/xyz
etc and so on.
Also to mount the tp-link usb it depends on how old the router model and firware is . If you have updated firmware and a fairly new (2-5 years old) router, they should work with smb2.0 but if they don't then try 1.0 for older ones. Newer routers do get newer versions as my tp-link old router updated to latest firmware support upto 2.0.
Hope that answers som queries and explains enough.
Have a good one ahead!