Hi aborges! I'm sorry to hear about your coverage issues. We definitely want to make sure we are doing everything we can to get you the best coverage possible. I found you a document that will not only show you how to test your coverage, but also how to request coverage if you are running on 2G or EDGE. Request coverage in your area
I hope this helps!
If your 4s is from AT&T and its unlocked it should have access to the 1900 band for Tmobile but it would have to have the correct settings. You can find info here http://support.t-mobile.com/docs/DOC-4771. I suggest to do the Carrier Update. Now whats that is done you can check your coverage in your area here http://www.t-mobile.com/coverage.html. Depending on what it shows than you can take the next step which is on the doc Jennifer posted..