I was annoyed by the Deco APP too. It provides remote management, which I believe that all data is forwarded through TP-Link's server.

My solution is running those devices as a Wi-Fi to LAN bridge, also setup my own NAT gateway (by bare-metal Linux, Openwrt... etc). Then blocking there devices from accessing Internet at gateway.

If I have more IoT devices at home, I will apply such policy to all of them.