@Harry_S
If you have firmware 1.6, it is supposed to alert you with the message when Ethernet link speed drops to 100Mbps:

If you have faster then 100Mbps Internet link, you can check Ethernet speed link between two Deco units by running speed test on device connected to Satellite Deco wirelessly or by Ethernet cable. Any speed test result above 100Mbps guarantees gigabit link between two Deco.
Deco M9 Plus hardware can deliver up to 650Mbps on WiFi connection and gigabit on wired Ethernet connection to it.