DIAGNOSIS The Deco X55 uses a single, combined network name (SSID) for 2.4GHz and 5GHz. Many older smart devices can't handle this and fail during handshake.
FIX You must temporarily disable the 5GHz band. In the Deco app, go to More -> Advanced -> WiFi Networks. Tap the 5GHz network and toggle it OFF. Connect your smart plug to the 2.4GHz network. After setup, re-enable the 5GHz band. The plug will stay connected.
IF THAT DIDN'T WORK Your main Deco might be too far from the plug during setup. Move the plug and your phone within 10 feet of the main Deco unit. Also disable cellular data on your phone during the setup process.
HEADS UP Never enable IoT Network or AP Isolation for these devices. It blocks local communication. If you use IoT Network later, you'll have to reset and re-add all smart home devices to that new network.