DIAGNOSIS The wireless backhaul connection is unstable, likely due to interference or marginal signal strength. Speed tests are a snapshot; latency and packet loss are the real issue.
FIX Reposition the satellite. Move it closer to the main unit, ideally with a direct line of sight. Avoid placing it inside cabinets, behind TVs, or on the floor. Check the connection strength in the app after each move.
IF THAT DIDN'T WORK Manually set the backhaul to use 5 GHz instead of 6 GHz. In the Deco app, go to 'More' -> 'WiFi Settings' -> 'Backhaul Connection Preference'. Select '5 GHz Preferred'. This band has better range through walls.
HEADS UP The 'weak connection' warning is about the link between Deco units, not your device's link to the satellite. An unstable backhaul will cause intermittent lag in games and dropped calls, even if download speeds seem high.