@Rafou 

Additional information : my switch is setup on automatic speed negociation and the Deco are working in Access Point mode.

  @Rafou 

According to hardware specs for switch Unifi USW-Pro-48 I can find on the Internet, that switch does not support 2.5Gbps speed. It supports 10Gbps or 1Gbps. For that reason BE25 is forced to connect to switch at gigabit speed.

If you want speeds faster that gigabit between Deco unit and switch, you'll need to replace BE25 with Deco model that has 10Gbps Ethernet port(s).

  @Alexandre. 

Thanks for the reply.

It does indeed not mention 2.5G and 5G on the tech specs. But it does offers me the 2.5G choice in the menu, which is strange (see picture).

I'll cross check with Unifi support.

Cheers