@Stefaan 

I have my mesh in similar configuration: M9 Plus Decos as Main and satellite nodes, running in AP mode. Main Deco node connected with Ethernet cable to Hitron Coda-4582 cable modem/router.

If you checked documentation, red LED means Deco has an issue. You have two options: return devices, or try to troubleshoot.

If you want to troubleshoot, I suggest you start with powering off all Deco satellite nodes and focusing on Main node. Red LED could mean many things, but it might be a sign that Deco can't access (ping, etc.) Web sites on the Internet it uses to verify connectivity.

Connect Deco Main node with Ethernet cable directly to Fritz!box router. Use cable that came with Deco devices. Try both Deco ports, one after another. Restart Deco after plugging cable to different port.

If that does not help and light stays red, next step: on your router, disable content filtering, parental controls, outbound connections limitations, everything like that - the objective is to allow unrestricted access from Main Deco device to the Internet. Disable firewall as well, if you can - who knows, it might be doing something with outbound connections, too.

If you are concerned with hack attack during troubleshooting - power off all devices connected to Fritz!box router (other than Main Deco) and/or turn off their WiFi.

If removing router restrictions helps and the light on Deco turns green, you can narrow down what exactly causes LED to go red by restoring your original router settings one by one. Restart Deco after each step, to make it check Internet connectivity right away.

As long as LED on Main Deco stays red, don't bother setting Deco mesh network.