@MeshTNN 

Deco mesh runs its own WiFi network, independently from ISP router. When you powered on Deco mesh it started with its "old" configuration settings, which include SSID and password.

Because these two WiFi networks are separate and independent (as if you had two regular WiFi routers in the house), it is recommended to turn off ISP router's WiFi. 

If you plan to keep Deco, it would be better if you could gain access to Deco mesh through Deco app, otherwise you'll be running Deco mesh without ability to manage it. In case you don't know what was the email address/password used for Deco app or can't use it any longer, I would recommend to reset Deco mesh units and start anew, as if you just bought them. 

The process of resetting Deco mesh units is described here, scroll down to "Reset a Deco unit" section: How Do I Reboot or Reset My System?

You must reset all Deco units, before starting new Deco mesh setup and configuration.

P.S. If you plan to give away or sell Deco mesh hardware to someone else, make sure you reset all Deco units before you do that.