Actually the rule is that your solar system must stop feeding power to the utility if the utility is down. You are allowed to run in isolated mode if your system supports that. Yes, even in Germany.
Every way I can think it through, isolated mode means your house is disconnected from the utility when you lose power, which requires more equipment than can be contained in a plug in solar panel.
Yeah the only code compliant way this is possible is if an additional sub panel is installed and maybe/probably a transfer switch that needs to be manually pulled in order to disconnect the sub panel from the main panel. Possibly OP is confused about running in zero export mode? Would love to hear how we could be wrong…