@saltbringer,

When it comes to disabling the SSIDs and Wi-Fi networks, this is only necessary to perform on your modem/router combo if you are planning on having the Deco control the communications on your network. Otherwise, you can configure as an AP, which will 'extend' the network created by the modem/router combo.

To set up a Deco into Access Point Mode, you actually have to complete the initial configuration and then change the operating modes. If you are still having DoubleNAT then you may not have confirmed that you are in AP mode. If you think that the settings are still an issue, you can always factory reset the device to configure from router from scratch. 

Unless you are using specific features on your G1100, I would recommend enabling the Bridge Mode on your modem/router combo and using the Deco in its default Router operation mode. When using a Deco in AP mode, there are certain features that are lost, and this can potentially provide a more stable connection if the devices are close together.