@Marrt did some more testing, IoT itself is working, but as soon as i enable device isolation, the isolated devices can't tolk to each other anymore which is something they should be able to.

I use the Deco IoT-Network and Device Isolation in order to shut out my IoT Devices (such as Shelly dimmers and raspberry Pis). But my Devices incrementally started to not be able to see each other....