From a webapp I can't connect to my own url

I think you are trying to access your own computer via the internet. This is called "loopback".

Some routers prevent loopback connections as a security feature. Most consumer-grade routers don't have any prohibition against it, it just doesn't work.

The reason it doesn't work is that the router implements request-answer algorithms, so is not programmed to handle the case where the answer is actually another request. In other words, the router implements the algorithm of request-answer rather than request-request-answer-answer.

This works from outside of your network when not in loopback.