Hi @sim0nw0lf,
The certificate in your browser ("Gültig ab 30.12.2020 bis 15.01.2038") isn't your Let's Encrypt certificate. First, it's self-signed (not issued by a certificate authority at all), and second, Let's Encrypt certificates are valid for 90 days, not 6225 days like this certificate. 
The certificate you're seeing was probably generated by the FritzBox itself or by some other device on your network, so you should check your port forwarding.