@Koennl
Hi,
Sorry to hear that you have some difficulties with the Deco M5. We can do further troubleshooting and try to fix it.
What is the current network topology/diagram? And what is the light status of the Deco M5?
With your computer connected to the modem directly, please check whether you can access the internet and the IP address obtained.
https://www.tp-link.com/support/faq/838/
To do a comparison analysis, please connect your computer to the Deco M5 and check the internet connection and IP address info as well.
Meanwhile, try to power cycle the modem and the Deco M5 to give it a go.
Besides, here is troubleshooting guidance you can refer to.
https://www.tp-link.com/support/faq/1454/
Thanks in advance, and have a nice day.