I could be wrong, but I think the AT&T $15 add-on is for calling from the US to the Caribbean. I think the only countries you could roam on AT&T prepaid are Canada and Mexico. You should call and check.
BTW, I have Google Fi and Tmobile/Sprint does not have signal in my building also, but US Cellular has super strong signal. And I was able to use an app, FiSwitch, to force Fi to use US Cellular.