To my understanding thats more a provider issue who need to be willing to support a niche product like the Fairphone like confirmed once for one of the Suize Operators
In Germany we dont have 3G since 2 years or so and it works here. Normally you should just be able to set the phone to 4g/3g./2G and the phone would just fall back to 2G for calls. So maybe your phone has a hardware defect or T-Mobile has to fix the 2G issue I would say…