Maybe you are trying to “hack” this into working instead of following some directions.
First of all, undo your tweaks to port forwarding and what ever else you did.
Secondly, you need either a Roku or a FireTV. I recommend a Roku, as I could only get one (of two) FireTVs to work right.Roku works fine.
OK, I will use a Roku as my example. So, you need the devices to both have the Plex APP installed on them. and set them to display the Plex app on TV.
You should be able to play onto TV from Plex app using the (Roku) remote
So, to get Alexa to join the party, you need to install the Plex SKILL in your Alexa mobile APP
OK, once skill is installed, you need to give Amazon permission to access your Plex account, and then you will be asked to enable Plex remote access
About now you should ask Alexa to Open Plex. It will ask you to select a primary player. You will be given some choices and one should be the name of your Roku., and hopefully you will be told it is. Now, tell Alexa to “ask Plex to play the Beatles (or whatever) ON THE ROKU.” Important, because although Roku is primary player, Plex still needs to be told where to play it. Also, ask Plex to play a certain movie.
If I did not leave anything out and you followed these directions you should be in business. Let me know what went wrong.
I got these directions from googling and a LOT of trial and error.