This was the original answer. Basically, it's how to tell if a device is accessible using Microsoft's SMB/CIFS protocols:
I assume you're talking about USB devices hooked up to the ASUS router. The first step would be to make sure the correct tcp ports on still open on the ASUS router. The easiest way to test this would be to use telnet or netcat (nc).
For telnet it would be:
telnet 192.168.1.1 139
telnet 192.168.1.1 445
(tcp ports 139 and 445 are the ones used for CIFS). Here's what you get if the port isn't open:
root@JarMini:~# telnet 192.168.1.1 139
telnet: Unable to connect to remote host: Connection refused
This is what you'll get if the port is open.
root@JarMini:~# telnet 192.168.1.1 445
Connected to 192.168.1.1.
Escape character is '^]'.
Note: You have to type Ctrl-] to "break out" of the telnet session if you get connected (or just hit enter a bunch of times and the SMB daemon will probably disconnect you). And apparently port 139 isn't necessary for "newer" versions of SMB. (My info on port 139 could be more than a bit dated.)
Netcat (nc) is a little simpler:
root@JarMini:~# nc -zv 192.168.1.1 445
Connection to 192.168.1.1 445 port [tcp/microsoft-ds] succeeded!
root@JarMini:~# nc -zv 192.168.1.1 139
nc: connect to 192.168.1.1 port 139 (tcp) failed: Connection refused
Unfortunately, there are several versions of Netcat. Ubuntu (and derivatives) use netcat-openbsd, which is what I used. Other linux systems may have different netcat's with different options. Thus telnet is safer if slightly messier.
The reason for this test is to tell whether the problem is SMB configuration or that the device simply isn't listening for SMB requests.
There could be potentially a problem with a network device in-between, but that seems unlikely.
In case you're interested the device I was testing above is a Mac, which I could connect to via CIFS from a Linux box.
TheTwilightZone jmitchel # uname -a
Linux TheTwilightZone 4.10.0-38-generic #42~16.04.1-Ubuntu SMP Tue Oct 10 16:30:51 UTC 2017 i686 i686 i686 GNU/Linux