Deco X60 mesh

Absolute non tech person, needing answers in 4th grade plain English. Trying to add X60 mesh to T-Mobile internet device, hoping to just plug and play. Deco app does not find deco device even tho iPhone recognizes and connects to the deco WiFi, and the deco device flashes blue to indicate ready to connect. Of course Ethernet cable already connected between T-Mobile device and deco.