I assume your building is made out of concrete and not steel/wood? Concerete and RF signals are like water and oil, they do not play well together at all. Heck, even some steel/wood structures don't do well with RF, especially if you're sandwiched in between floors. I don't know that there's anything we can offer as far as help goes. Your problem is all signal based due to the structure you live in. Now that I think of it, Sprint has an in-home repeater you can buy. I know you're not on Sprint, but maybe AT&T has the same thing. It's essentially a little cell phone tower for your house. Instead of connecting to another tower, this repeater connects to your Internet connection and transmits your voice calls/texts that way.
