When you say open a port you mean forwarding connections to that port to your machine inside your lan?
Next steps I would take are to verify you can access the port within your lan
I.e. if your machine has a local IP of 192.168.1.23 and your service is listening on port 4200 try connecting to 192.168.1.23:4200 from another device on the lan, or even from the machine itself although I'm not sure that's always a good test.
You can also try nmap to scan you lan or netstat on the host to check what ports are in use.
If that fails you may have a restrictive firewall on your machine blocking inbound connections. A quick check to see if this is the problem is to disable the firewall entirely, just remember to turn it back on if you need it!
If you can access it locally on the lan, sniffing traffic with Wireshark may help debug the issue. You should be able to see the router sending forwarded traffic into the lan. If the configured IP address is not known to the router you may just see arp requests for who has [IP address to forward to]
For me the tinkering is part of the fun, so old beat up ender 3s are perfect. If you want something ready to go, even a brand new ender 3 won't give you that 🤣
It's okay to cry, but also keep going until you figure it out, and watch freecad tutorial videos. I think learning how to cad on freecad is a nightmare, but once you know how things are supposed to be built it works well.
Agreed it sounds like op wants luks. Dare I say if you want bitlocker for Linux, it's luks.