Set up a reverse tunnel to the outside box that you want to get in from.
For example, from the inside machine:
ssh outsidemachine -R 2222:localhost:22
Then on your outsidemachine:
ssh -p 2222 localhost
Maybe run top or something just to keep traffic going across so that firewalls don’t drop your connection.
I use Jellyfin which is similar to Plex. I have it on a Raspberry Pi 4 8 GB. It’s perfectly fine if I’m sending H264 but most modern browsers do not support H265 so it forces the server to transcode. That will consume almost all processing power if it’s CPU-only and is a very slow process.