Right, it should be pretty straightforward. And looking over your screenshots I seem to be similarly configured. All devices connecting to the X750 point to the same IP range as the router, what shows up in the LAN IP screenshot above. So, the network traffic should just pass through to the phone. But it doesn’t!
I don’t believe that the tethered phone is the problem since tethering works just fine on my windows laptop. Using ipconfig I don’t see anything different from what is shown above in the screenshots.