DIAGNOSIS This is almost always a communication hiccup between the app on your phone and the Deco units. Your internet is likely working because the Deco's connection to your modem is fine, but the management path back to your phone is broken.
FIX Don't touch your modem or Deco units yet. Start with your phone. 1) Force-close the Deco app completely. 2) Turn your phone's WiFi off and then back on. 3) Re-open the Deco app. This clears the local connection. If it's still offline, check your phone is connected to your Deco WiFi, not cellular data or a neighbor's network. If all that fails, restart the Deco system: unplug the power from all Deco units, wait 30 seconds, plug the main one (connected to the modem) back in first. Wait 2 minutes for its light to go solid blue, then plug the satellites back in.
IF THAT DIDN'T WORK There might be an IP address conflict. Log into your modem's admin page (if you have a separate modem) and check its connected device list. If you see the Deco's IP address listed for something else, you'll need to restart the modem as well to clear its DHCP lease table. Unplug modem, then Deco, wait 60 seconds, plug modem in, wait for it to be fully up, then plug Deco in.
HEADS UP The app uses your local network to talk to the Decos. If you're trying to manage them from outside your home (remotely), that feature requires the Deco to be logged into your TP-Link cloud account. Double-check you're logged into the same account in the app that you used during setup.