contact T-Mobile through a phone call or social media and they can find out what the best option of a coverage device would be for you. I signal booster maybe all you need. And there is usually a $25 deposit but not always.
Can anyone else make calls from inside your building? If not, it's definitely something like the building's materials blocking the signal (since you have no problem getting a signal outside). Keep in mind that cell phone boosters can't create a signal out of thin air so you'll want to test the signal in your building. This is one method to do so short of bringing a technician.