If you encountered a problem accessing a network drive (through mapping in Windows) after an upgrade of firmware is performed on the router, the following could be one of the reasons that may have caused the headache.
In this case, the router is ASUS RT-AC68U. The network drive is a USB disk attached to the router. The drive is shared using Network Place (Samba) Share. After an upgrade of firmware, the “Device Name” was reset to a default value causing the accessing/mapping problem. Just update it to the custom name that was set before and the problem should be resolved.