Netgear X6S EX8000 is not connected to the router (allegedly)

A couple things. 

1. 2.4ghz is more sensitive to interference than 5ghz. so even if the phones are the only devices on the 2.4ghz, if you're in the range of other networks, they can create interference. 5ghz doesn't broadcast as far or through as much materials so it doesn't interfere as much. And because its faster, when its broadcasting, its gets the data through faster so causes less interference because of that as well. 

2. how far from the router is the extender? Whats in between the 2? (walls and what they're made of)

3. are you setting up using wps or the installation assistant? 

4. did you check off the "extend 2.4ghz" during setup?  I usually uncheck this option. Reason why is then it doesn't give the extender the option to fall back to the 2.4ghz if theres a problem with the 5ghz. With 2.4ghz suffering from more interference, its better to have it use the dedicated backhaul and to get that setup properly. 

5. if you are using the 2.4ghz and for some reason thats operating as the backhaul, the throughput cuts out 50% as well as latency gets increased. Another reason to not let it use the 2.4ghz as backhaul.