Help Needed

I need your help with my service. I had bad coverage in my area. I did replace two of my phones to new 4G phones (Smart Phone), but still the service is bad and I do not know what to do.
Second, since T mobile changed to unlimited plans I was told I can not do any changes to my current service unless I upgrade my plan. Is this correct? cause I was told only T-Force team members can do these changes.
Your help in this matter is greatly appreciated.