I live rural and I have pretty bad 4GLTE signal, using a Galaxy Note 9, and I wonder how I can contact T-Mobile to maybe improve their singal here

A lot of things can factor into how well your devices works with the coverage available.  Considering you are not a T-Mobile customer, your experience on an company that uses aspects of T-Mobile's network more than likely will differ than that of someone who actually is a T-Mobile customer.  Also, the S9's are quite old!  Radios in devices have improved a lot over the past 4 years.  Have you considered upgrading your device and potentially trying T-Mobile as your service provider?  Cell Boosters are no longer issued by T-Mobile.  T-Mobile customers can get a Cell Spot, which uses their home internet connection to provide a cell signal for them to use to improve indoor coverage.