Can't access password protected smb share
I had the same issue with my Netgear Nighthawk R8500 until I reduced the router's password to 14 characters, as suggested by user137327. Since the router's password is also used as the Samba password, perhaps the cause is related to the legacy of Samba and Lanman password limitations:

"passwords are limited to a maximum of only 14 characters"

"Many legacy third party CIFS implementations have taken considerable time to add support for the stronger protocols that Microsoft has created to replace LM hashing because the open source communities supporting these libraries first had to reverse engineer the newer protocols—Samba took 5 years to add NTLMv2 support"