Can you share what you asked when calling Straight Talk or know what they did on their end? I have the same phone, but lost 4G completely when activating with Straight Talk. All I get is 3G. I've called support twice, and all they tell me is that it's my phone and I need to contact Sprint.
Had 4G when my service with Ting stopped, had 4G when my phone was trying to activate with Straight Talk (no provider), but not after activation.
Any help here regarding what they did on their end would be greatly appreciated.