Domestic SIM unlock Galaxy s6 Edge+

hey guys, im new here. trying to look for some answers because i hit a dead end. Does anybody know how to domestic SIM unlocking the Sprint Samsung galaxy s6 edge+? Story is i bought the phone on ebay, guy said it was unlocked and I did a lot of research on putting the sprint phone to straight talk (t-mobile or at&t or verizon). I thought it was unlocked and everything would be fine. I really dont want to return the phone because I got it for cheap and already bought the parts to fix it, its just cracked. I called sprint and they gave me the MSL code and tried to unlock it but the phone would not respond. So then i guess they put in a request to domestic sim unlock it and they denied it because they said the account was in collections. Is there anyway to domestic sim unlock it for it to work on t-mobile or at&t without going through sprint? but dont try to rip me off because its just straight tech which im at a dead end now.