@Alexandre. Yes indeed, my ethernet port 5 on my isp router is 10Gbps but selectable 10,100,1000,2500,10000 full/half. I've forced it to 2500 full. Still have the message on deco.

@MarkMol I am also getting this message. Deco xe75 2.5GbpS port connected directly to 10Gbps port on ISP provided router/modem port forced to 2.5Gbps/Full. ISP provided bandwidth is 1.5Gbps up/down....