Hey @Bluestringer,
A few things could be at play:
1 - You indicated that you are using a Deco W3600 which does not have the ability to broadcast your entire gigabit connection over a wireless connection, especially if your backhaul is in use at the time. Saying that, you should be able to achieve significantly higher speeds than what you indicate, especially if you are able to connect to the network via a 5GHz connection.
2 - For your ONT, can it also operate as a wireless router? If so, there are a few settings that you would need to adjust on either the gateway or the deco network. What ONT are you connecting to? Who is your ISP?
I would recommend first testing the connection with a wired device connected to the main node, as this would let us know if the main network is receiving the whole speed from your ONT/ISP.