Samsung Galaxy S5 — (PLEASE HELP) SAMSUNG GALAXY S5 WON'T CONNECT TO STRAIGHT TALK

Hello, Im brand new to the forum world, So I'm sorry if I'm doing this wrong.. I have a SAMSUNG GALAXY S5 that I purchased used from a Facebook yardsale page and I have done everything known to man to try and apply my number and service from my old straight talk phone to this one, Including spending 2+ hours on the phone with ST Tech Support which was no help.. So the problem seems to be with the SIM, when I insert a Tmobile SIM it will connect and I even get (Welcome) messages from ST, But incoming text is ALL I can do, No Data, Cellular,MMS Etc... So ST told me to insert the provided AT&T SIM (which apparently will work on any unlocked phone) and it now does absolutely nothing... I can't change my APN settings AT ALL with any SIM I use, My Network Operators is Greyed Out and everything under phone info is "UNKNOWN" (Network, Phone Number Etc..) I have an active unlimited service card with 19 days left and this is the second month I have paid for service and I haven't been able to use the phone ONE TIME.. So If any of you Tech Guys out there could help me PLEASEEEE, I would greatly appreciate it.. I can provide any additional information that's needed thanks you... Josh