Sprint has a device called the Airave they can send you to help with indoor coverage. Sometimes its on back order, can take a few weeks to get, but worth the wait.

But don't waste your time, do your homework first:
Check Is AIRAVE service available at this address? to see if it will even work at your address.
Next, make sure you have high speed internet (DSL or CABLE). Satellite or Dial Up WILL NOT WORK, so don't bother.
Next, be prepared to wait. Call sprint at *2 , tell them you are thinking of cancelling due to coverage issues while at home. They will transfer you to account services, and they will order you the device, mention a rep told you about something called the "airave", and ask if you qualify for that instead of cancelling.

Also tell them your issue is not resolved, until they do.

See More: Improve in home coverage complaints