This was a router issue with my MAC Address filters. Silly me banned my own laptop from using the router.
On the SMB issue, I managed to connect using cifs. Here's my command-line mount:
sudo mount -t cifs -o username=guest,guest,vers=1.0 //router_ip/share /home/user/mountpoint
At least I am now able to connect and transfer stuff to the router's external drive. I had similar issues with Linux Mint Rebecca (17.1) and the solution was to roll back to version I had back in 2016 (4.3.9).