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
My concern was with the data download speed and it turns out my concern was misplaced. The Sprint/Clear 4G WiMax advertises up to 12mbps max and 2-4 mbps typical, which again, is must faster than dial-up; while the DLink router can handle up to 54 mbps. So data speed should not be a problem.
If you have a problem with it, I would do as postyle did and have Clear provide the modem and router... then you can require them to make it work or you can withhold payment.