Different countries have different laws pertaining to the internet!
I'm not surprised to hear that your German card won't work in France!
One possibility,
The phone company that activate your German Sims card may not or may not be allowed to do business in France!
You are trying to take and use a companies agreement across a international border! Before you try to do this you should check and see if the company does business in that country!
In otherwords; you may not be able to fix this! You may have to only use the Sims card in the country from where it was issued!
Another possibility. Check your German Sims card and see if it was set up for roaming in France!
If it was not that could be why you cannot use it there!
— modified on Sep 2, 2018, 3:41:25 PM