Coverage - what is the plan for my area?

I've been suffering with very poor T-Mobile/Sprint coverage for years in Byron, MN.  The cellspot T-Mobile sent me helps me make calls, get data and send text - using my wifi router to provide the connectivity.  However, if I leave my home, my cell phone coverage is very poor.   My Verizon work cell phone works great!    I've called T-Mobile customer service and have to use my Verizon cell phone to complete the calls!  What is the plan to improve coverage in zip code 55920 so I can use my cell service without being connected to my cellspot?   I'm so frustrated that I am going to switch to Verizon if there is no plan to improve coverage.