@IgorRJ
You have listed Deco M5 with firmware 1.6. See this forum topic: Deco M5 1.6.0
First post, New Features, Item 6:
6. Added a prompt when the Ethernet link rate goes down to 100Mbps.
Note: Deco M5 supports a maximum link speed of 1000Mbps, but due to the quality of the Ethernet cable, the link speed of the connected device, the flow control, and power-saving settings, etc., the link speed can sometimes drop to 100Mbps or even 10Mbps. This can affect your download and upload speed if your bandwidth is more than 100Mbps. After installing firmware 1.6.0, you will receive a prompt in Deco App when the Ethernet link speed is not 1000Mbps.
Have you checked Messages in Deco app?
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Also, to check the speed between Main Deco and Router, you can use "Test Internet Speed" in Deco app, if you haven't done it yet. That will report Internet speed from Main Deco to Router and Internet.