Follow the below steps to allow CSF through CSF.
Step 1: Open the Terminal or Putty.
Step 2: Connect to the 'server' using SSH.
Step 3: Open the csf config file using the command,
Step 4: Under the heading # Allow incoming TCP ports, there will be a list of port numbers like as displayed below.
TCP_IN = 80,443,22,21, .
Step 5: Add the port number after the final digit.
Step 6: Finally, enter the command
to restart the 'CSF'.