RDM understanding

Was RDM activated in the Shell Settings (Network Settings)? The communication with Sensor is not using RDM.

yes

There are also RDM setting in Setup > User > Output which must switch on.

On the Element, In the Shell, On the Network Page, Under Interfaces, for the Network Card you are using, make sure that RDM is enabled.

Also, while you are in the Shell, you can go to the Maintenance Page, then Launch Concert and perform a Network Map.  If RDM is configured properly in the rest of the system then you should see the fixtures show up connected to the Gateway.  This will confirm if it is an Eos setting issue or a larger issue.

yep its set up like that

but i did not have to do anything for the other Element.  no concert or mapping.  I have the other element plugged into a Ethernet hub, and my spare NET3 node into that and the Luster into the port 1 output configured as universe 2 and the RDM works fine

The Concert part is to test to confirm if the Element (as a computer) is connecting to the Gateway and getting the RDM traffic outside of the Eos/Element OS.

Based on your two comments you "Backup" Element can see (over RDM) the Fixture either through the Local DMX port or over a network and Gateway?  If I understand you correctly, this is not the same network as the "Primary" Element - is that correct?

Forgot, this is only for Hardware Outputs.

correct.  the backup is just another Element in our shop.

In Concert, take a close look at your 2 port Net3 gateway and make sure RDM is enabled in the gateway.

i dont see concert on my Element

i dont see concert on my Element

It should be in the shell on the maintenance tab