Do you have an Eero, Google Wifi, Netgear Orbi or other mesh network wifi router system? These can make it tricky to connect your Deebot to your phone. They use a single wifi signal that allows devices to live on both the 2.4 and 5 GHz radio frequency. This ensures your devices are able to move along eero's mesh efficiently, delivering the highest results to the devices throughout your entire home. But - it also means you can't designate which band you're using.
Since the Ecovacs Home App only connects to 2.4 GHz band - you'll need to connect to that band on your mesh wifi network. To do this - take your robot and move far away from your router node. Get as far away as you can while still maintaining connection to your wifi. The 2.4 GHz band travels much further than the 5 GHz band - so there's a great chance you are now connected to the 2.4 band.
Now follow the instructions on the app.