@Jabronismacker,

Our Deco's are all meant to work together, the only thing that can be limited when adding older decos is some of the newer network management features as some of the features require all the nodes on a network to support the feature. From what you are describing, I am led to believe that the issue is with the S4 or how it connects to the network. If all the nodes disconnected at once, I would be more inclined to think that it is the XE75. 

How do you have your Deco S4 connected to your network? It sounds as though it is using a wireless backhaul to the XE75. If the units are far apart, you may try to move the satellite node closer, as it could be losing its connection due to an added source of interference or if the satellite is placed towards the end of the range of the XE75. A lot of people try to maximize the area that their network covers by doing this, and realistically you will have the best experience and range when the satellite nodes are placed at a midway point. If it is impossible to move the node closer, you may try changing the S4 for the other XE75 satellite, as the XE75 will be more powerful and may be able to better handle whatever is causing the disconnect on the S4

You can find the hardware version of your Deco on the label on the bottom of the device. I would manually check your product's download page to make sure that the S4 is using the most up-to-date firmware for your version, as stability updates are released semi-frequently and it could be possible that your device is not autoupdating.

  @Jabronismacker,

Your hardware version can use the same firmware as the V2. It looks like you are using the most up-to-date firmware. If the device is that close to the main node, then the distance should not be an issue at all. 

I would recommend resetting the S4 and reading it to your Deco network, that way any conflicting settings or configurations will be overwritten to work with your current network setup.