which wireless modem are you attaching to the D-Link router?
In many cases people are plugging a "4G" USB modem into their router without considering that the USB modem itself has the 4G antenna built into it (not the best configuration for RF performance/propagation).
Think about moving the router itself (with the 4G USB modem/antenna) to a window, preferably a window facing towards the "tower".
Some of the integrated modem/routers (4G built into the router) offer the capability to add an optional external antenna so you can place the antenna in a more optimal location for the signal.
Sprint/Clear use spectrum in the 2.5GHz range, so indoor propagation is not optimal