Hi @Christoph_Gobel and welcome to the forum.
Make sure that vSwitch is not active on the NAS. Read also https://roononnas.org/en/synology/:
Additionally if needed: Power cycle all devices in question and in between, including the NAS, output devices, network switches, bridges repeaters, routers and so on.
If it’s still not working please add more details about your (network) setup, what troubleshooting steps you’ve already done and if they changed anything (even if only temporary).