If you have already checked the three suggestions from the error message, the most probable cause of this error is that the first 3 numbers in the board's IP address do not match those from the host computer's IP address.
If you changed the board's IP address to match the host computer, be sure to update this information in the model's Configuration Parameters, under Code Generation->Interface, in the edit box for "MEX-file arguments".
To resolve the 'ext_comm' error, you can change the IP address of the board to match the first three numbers in the IP address of the host computer. Once you have logged in over a serial connection, you can use the the ifconfig/ipconfig command to change the static IP address.
Once you have done this, you should also make sure that you change the IP address in the model's Configuration Parameters, under Code Generation->Interface, in the "MEX-file arguments" box. Be sure to surround the IP address in this dialog box with single quotes, and try running the model in External mode again.
Hope this helps.