That's a question ChatGPT would excel at... It sounds like it might be a big task for your shoes, but here goes (100% organic reply, mind you):

• get a Raspberry Pi, install your web app, make sure you can reach it on the LAN
• forward the port your web-app is running on to the Raspberry Pi: app is now reachable under IP:port from the internet)
• set up a domain with a provider that supports dyndns, set up a job on the Pi to regularly check in with the dyndns provider and tell it your current WAN IP: app is reachable on your own domain; success

This is leaving out any security measures and you will want to take care of those before opening your home LAN to the world! Have fun learning!