Page 83 to 85 of the manual will be very helpful. Explains the port forwarding options.
The External IP description reads “You may optionally enter Addresses of External devices on the public internet”.
So like I said leave that blank, do not enter an IP Address there.
As for the “Local IP Address” of the Tablo, it is not 192.168.0.1 - it is something else in the 192.168.0.2-254 range.
Good luck!