Python's webserver is meant to be a quick option for testing, and shouldn't be used for a publicly accessible website. You'll want a proper webserver for that.

You would need access to the router for the port forwarding, so if you can't access it, you'll need an alternative option. The next best option is a commercial reverse proxy, most commonly that's Cloudflare's "tunnel" product. Essentially, Cloudflare acts as a man-in-the-middle, forwarding from a public address directly to your local server. It's pretty easy to set up and it takes advantage of common router settings to open ports from the inside, where the router's firewall rules are typically quite lenient. This also works with CGNAT, so no need to figure that out. The downside is that Cloudflare gets access to all the data that passes through them, even passwords. So if your website happens to have a lot of sensitive data, you'll need to decide whether you trust them with that data. They do not get access to your normal web browsing or anything, just the data passing between the public address and your local server. Another potential downside is that they prohibit media streaming, so if your website serves a lot of videos, you'll want a different option.

Google Cloudflare tunnels homelab and cloudflared for more info. There's a bunch of YouTube videos and tutorials on setting it up. It's not perfect, but it's a free and easy way to selfhost from a limited network environment.