first of all i wanted to clearify, that i'm not asking, how to access a raspberry pi from the internet. I know, that I have to use a DynDNS or the IPv4 if it won't change. But I can't use this.
So I will explain my problem: A friend of mine and me are working on a project. We are creating a automatic watring system for gardens or plantations and we have a apache on the pi, so you can access the pi from the home-network. But we also want to access the pi from outside, but without using a DynDNS, because we want to make it marketable, so you can't give every pi a DynDNS.
At networks, where the IPv4 won't change it is not a problem, but usually the internet provider change your IPv4 nearly every day. We also thougt of building a own small server with old PC-Components of our parents, but we don't know how to deal with the changing IPv4. Theoretically we could build a own small server if it is a solution.
So I wanted to ask you, how to access a raspberry pi with changing IPv4 without DynDNS.
(Sorry for my bad englisch)
Your bigger problem is that 99% of consumer devices are behind a router. You will need a solution that opens a connection to your service and waits for commands. This concept exits with tools like netcat or ngrok.