Quick update.
I found out that I can have a free domain name with my internet provider. So I created a SSL certification.
However, I can subscribe to the webhooks, but it seems that Twitch does not answer the “hub.challenge”, or I am listening wrong on my server. Thing that disturbs me a lot is that it works for the “follower” indication with http, but does not once I use https.
I can access my domain name and my equipement fairly easily outside of my local network. I just do not get why it is working one way but not the other.
Thanks for the help !