Need advice. What and how to do it (if there is anything that an be done at this point).

Got the phone from Swappa to replace a working S6edge,, Switched the SIM and upon activation realized that data does not work, called Red Pocket support they did some changes and lost text, then they asked me to took at ADN and we changed the settings as requested but lost text and voice and got data. They tell me is a firmware issue. This is unlocked phone G970U1. Trying to make a call, send me directly to VZ, trying to call the phone send me into a voicemail. The answer from RP tech is will send you a GSMA sim card and then reconfigure the phone to work on GSMA network. Any help, what I can do now? What I need these guys from RP to do ?
Trying to go back to my old S6 with this old SIm card, now there is no voice with the S6. Major inconvenience. Please help