Hello Manfred,
Thanks for your explanations. All your arguments sound reasonable to me that I seem not to have a real IPv4 address. Unitymedia sold it to me as IPv4 
:
I got a written confirmation like this '… und IPv4 Anschluss bleibt erhalten".
What does it mean to solve my problem?
Is there any IPFire core 155 user with Unitymedia as ISP, a Dual Stack Lite connection, and an ATA SP112 working behind the IPFire firewall?