So I could not stop thinking about this issue and why it was only happening with my ISP at home.
I looked for help in a subreddit for my router. And it turned out that I needed to change the MTU size of my network device.
I wonder… I have been with this connection for five years and with my new router for a couple of months. And it never happened to me with any other side.
So I wonder if this is something that needs fixing by fly.io. If I remember correctly, they operate their own data centers.
I am really worried about building my next startup on the infrastructure of fly.io, because also others could be victims of the same issue.