How to unlock IPhone 6s Plus from StraightTalk

Hello! I have purchased iPhone 6s Plus from Walmart in Georgia, USA during my business trip. When I came back to Latvia, I realized that the phone is locked to Straight Talk even though I asked the sales assistant for the unlocked phone which I can use in Europe. Tried to contact Straight Talk, they asked 90$ for unlocking which I agreed on since I have no choice but afterwards they did not answer at all. Is there any other way to unlock the phone?