@gunna 

Router mode is a mode where the Deco acts as a router. This means it generates its own Private LAN, has DHCP enabled to automatically assign IPs to clients connecting to this LAN and can do all of the features you would find at a router-level device, such as QoS, Port Forwarding, and Parental Controls.  When you place a router in AP mode you are disabling all of this, so that is NAT-Free mode. Unfortunately, NAT or this private LAN is needed for these router-level features to work. 

The best solution is to work with your ISP and have them Bridge their gateway or provide you with a Modem only device so that you can take advantage of all of the rotuers features.