@mkdeco 

Welcome to the community!

The sudden cap at roughly 10 Mbps is certainly peculiar. What are you using to test the speed? For example, are you testing over Wi-Fi on a smartphone, or are you testing via Ethernet on a PC that's directly connected to your Deco?

You mentioned not seeing the QoS option in your Deco app. Just to be sure, this option is typically found under More (4-squares icon) -> QoS. 

If it's not QoS, I'd suggest bypassing the main Deco and connecting a device (e.g., laptop) directly to your Internet source (modem, etc.) and running another speed test. If the speeds are still capped around 10 Mbps while connected directly to the modem, I'd contact Cox to see whether they can diagnose your line.

However, if the speeds at the modem are at or above 1 Gbps, then I'd start looking at swapping one or more of your Ethernet cables, specifically the one between the modem and your main Deco, to rule out a failing cable.