@darnoc23 

There are 2 ethernet ports on each Deco units. May I know how many Deco do you have?

Per you said, you can connect the Deco to the C3150 to get internet or configure the Deco as main router and connect C3150 to the Deco to extend the network. 

While the devices connected to the C3150 won't be able to utilize the Deco mesh network, instead, the devices connected  to the Deco itself can. 

Both options can be used to extend the wifi range, you can choose one of them based on the actual network request. 

@darnoc23 

With Deco connected to the C3150, you can keep those connections already existed, first, you can configure the Deco as a router mode, then you can use the advanced features, like parental control, while there will be 2 networks; devices connect to the C3150 and Deco cannot communicate with each other; or you can configure Deco as access point; the mesh network still works, and all the devices will be managed by the main router (C3150）, but the advanced features won't work. 

If you connect the C3150 to the Deco as a slave unit, you need re-build up the current connections; and the Deco will work as the main router. 

Good day.