How Do I Repair My International Model Phone?

I purchased a Samsung Galaxy J7 Pro off of Amazon. This model of the phone is more common in Asia, and the wrapping was in Japanese when I first received it. It's a nice phone, and it was working well until one day I noticed a growing purple splotch on the screen. The discoloration has begun to spread, and everything that I have read says that I need to repair it. When I went to approach Samsung about their two year warranty for my phone, I found that it will not let me register my product. It says that my serial number is invalid, and neither of the IEMI numbers is accepted. I tried to call with a representative, and all they had me do was give them the numbers only to tell me again that they were invalid. The splotch on the screen, though it does not affect the sensitivity of the affected area, does seem to be getting bigger. What should I do?