iPhone XR screen problems now dead

I bought my iPhone XR about 3 months ago through Rogers on an upgrade plan. All the sudden one day the screen starts turning green and displaying lines. No worries phone is under warranty I thought, YEAH RIGHT! I take iPhone to Apple for service and they come back with "we cannot fix because it has unauthorized modifications", not providing much details other than that. So, take it back to Rogers and they say it's Apple's problem. Apple says take back to Rogers. Here I stand with a dead iPhone extreme frustrated. Apple says the repair depot has the final say and they will NOT fix. What's worse, the phone would at least boot up before and now it doesn't.

Why, just why would I modify an iPhone out of pocket when it is under warranty?