Lied to by a supervisor and no help afterwards

Bought a Dell S2716DG monitor, and was a couple days past the 30 days return date. I called Dell due to an issue that caused a vertical line to appear on the middle and the right side of the monitor occasionally after it enters power saving mode. I talked to a supervisor who couldn't offer me a warranty exchange for the issue, but told me I was welcome to return the monitor even after the 30 day period, as long as I call another number, and reference a case number. Called and referenced my case number, but the representative told me I wasn't able to return my monitor, and there was no supervisor to talk to me further. 

I am seriously disappointed in your products and service. I informed the previous supervisor that my monitor was past the return date, and I would be happy to pay a restocking fee, still he approved my return. Why does one supervisor say one thing, and another employee say another? I won't be buying any further Dell products in the future if this is the service you offer.