I have the same issue previously. Gotten a second hand iPhone 6S plus from mudah.my. I didn't check upon COD. When I reached home, put sim card card, cannot work at all. For maxis centre, change new sim card also same. No signal.
Then since it's under warranty, I just walk into apple warranty centre. Then check my iPhone and took my phone for further examination. One week later, then just replace a brand new one.