It is a 10 Mbit/s half-duplex issue

JJG1992 wrote:

It is a 10 Mbit/s half-duplex issue

Appears the Ethernet ports on "modern" (and fast) routers, network devices like Mesh satellites, does fail on the auto negotiation on 10Mbps half-duplex only devices like the tado Internet Bridge  - and probably also on 10 Mbps full-duplex,  100 Mbps half- and full duplex devices.

The 'Router' LED on the Internet Bridge is blinking. What does it mean? | Help Center for tado° V3+ and earlier devices 

Many popular ISP provided modern and fast routers (Salt Fiber, BT Hub, Sky Q Hub, Virgin Media, and EE Smart Hub routers for example)  are affected, too:

What do I do if I have problems connecting my Internet Bridge to the router for the first time? | Help Center for tado° V3+ and earlier devices 

The workaround suggested is deplyoing an unmanaged network switch: "if you’re still having issues, consider buying a 100/10 Mbit/s network switch"

By rule of thumb, all Netgear switches with Gigabit ports can be configured to 10 Mbps-half. For example GS108EPP:

This does not apply to the MultiGig and 10 GbE models - very similar to the Orbi 8 and Orbi 9 Series - where both auto and manual (where avaiulable) config is limited to 100M Full, 1G Full, 2.5G Full:

Very useful information, thank you JJG1992​ !

And JulienR​ ... once more the in-line imgaes don't show up without a forced browser reload  8-(