@DLPlatt 

Technically yes, but its not recommended.  They would not be the same network and it would do more "harm than good".  You clients would likely become confused as to which AP to connect to, the Deco or T-Mobile.  They would either constantly jump between the two or just not connect at all.  What would be best is to disable the wireless network on the T-mobile router or even operate it in Bridge mode.  Then run the Deco as the only network in your home.