Trip.com complaint: Refuse to change my seat

I brought 2 air tickets Kuala Lumpur to Dubai , Singapore Airlines. I know that my return have to amend and flexible, so I brought free for cancellation at premium price . When I try to change , I was charged for the changes.

As I had checked in online and my sear were allocated differently, I wrote a request to Singapore airline but as I brought from Agent, they requested me to go back to my travel agent to arrange , so I chat with trip.com. Initially they wanted to charge me more fees, I told them that I am not purchased a budget airline tickets , how they have to charged me , then she keep quiet and agree to help me to arrange , after 2 hours , I received an email from Trip.com that they unable to assist as checked in not open yet . How I can checked in and allocated wrong seats if it was not open, it should open 72 hours prior to take off.