@Dancaw,

As you mentioned, the two systems(Deco and Aginet) are entirely separate, and you are best suited to go to your ISP to figure out what they recommend for your network. Saying that:

First, how have you been testing your network speeds? If you have been testing wirelessly, it is not uncommon to see a far lesser number than what you would expect. When testing your network's speed, it is always recommended to run a wired speed test to the main deco node.

Testing this way will give you a much better idea of the speeds your Decos are capable of achieving. This will also prevent the wireless network being broadcast by your Gateway from interfering with your tests. I would also make sure to try different Ethernet ports and use a different network cable between your Gateway and your Decos.

If you are unable to put the Aginet modem/router into bridge mode, the loss would likely be a result of having two routers functioning on your network.

In this case, your best option is to put your Decos into AP Mode, which allows your Aginet router to manage the entire network's connections, while your Deco Mesh System still provides your actual Wi-Fi. 

Looking at the specifications for the EX820v, it is compatible with EasyMesh - so while your current Decos are not able to mesh directly, there are a variety of EasyMesh devices that can mesh with your Aginet system directly. Most Archers and Range Extenders are now compatible, so even if you decide not to use a direct Aginet solution to extend your network, there is a high chance that you will be able to find a solution that can.