Friends unable to join my server

I have recently started a new Minecraft server using the truenas plugin. I followed a tutorial to set up port forwarding for the static IP given to my server and logged in for the first time successfully. I then gave the IP to my friends and all of them came back with the same error, unable to connect. I used the open port check tool to test if the port was actually open, according to the tool it was not. I have no clue why this is happening and any help regarding the topic would be much appreciated.
Thanks in advance, ELP