@Francois4NL 

I see two possible reasons for what you described: networking switch is not compatible with Deco Ethernet backhaul or powerline adapters are not compatible. Worst case, could be both.

Test the switch first. Bring Deco 2 to where switch is and connect it by short Ethernet cable directly to switch.

Main deco ---switch --- ethernet deco 1

                           L---deco 2

See if Deco 2 reports Ethernet connection. If it does not, you will need to get different switch. More about that in the following document: General questions about Ethernet Backhaul feature on your Deco. See Q4 and Q5 in that document.

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If you tested switch and found it is compatible with Deco mesh, it is possible powerline adapters are not. Not every brand of powerline adapters supports Deco mesh Ethernet backhaul protocol.

What brand and model of powerline adapters do you have?

I just had conversation in different forum thread on that same topic, so I'll just paste here what I said there:

Not every powerline adapter is compatible with Deco Ethernet backhaul protocols. Choosing TP-Link brand is a good idea. Avoid powerline adapters that have more than one Ethernet port, for some reason those with single Ethernet ports are more likely to support Deco. You may need to return adapters if you can't make them work with Deco, so get them from place with return policy.

I have pair of powerline adapters I tested my Deco with and mine do work with Deco. These have been in my house for few years, not the latest model, but you can still find them for purchase on the Internet. It is ZyXEL PLA5215. If TP-Link brand adapters do not work for you, try these or similar but newer model.