At what level do ISPs control ports?

https://beehaw.org/post/6960674

At what level do ISPs control ports? - Beehaw

I’m aware most ISPs do not allow for port 25 to be open for email use outside of business licenses, but at what level is that controlled? Can I get around that by owning my own router? Owning my own modem or ONT? Or is this just a thing they mystically control further up the pipeline that a relative layman such as myself can’t get around?

Well, I can say that T-Mobile 5G Home Internet completely blocks all incoming port traffic.

So, you have to get creative with stuff like WireGuard & Cloudflare tunnels to do some stuff properly.