DIAGNOSIS The Deco mesh is stable, but the connection between your main Deco and your internet modem is failing. This points to the WAN port or modem compatibility.
FIX Power cycle the entire chain. Unplug your modem, then all Deco units. Wait 2 minutes. Plug in the modem. Wait for all its lights to stabilize. Plug in the main Deco unit. Wait for solid blue. Then plug in the satellites.
IF THAT DIDN'T WORK Check the Ethernet cable from the modem to the main Deco's WAN port. Swap it with a known-good cable. In the Deco app, go to Internet Connection and ensure the connection type is correctly set (usually DHCP).
HEADS UP Some modems, especially from ISPs, have built-in routing that conflicts with the Deco. You may need to put your modem into bridge mode. Contact your ISP for instructions specific to your modem model.