Mytrip.com complaint: No change of flight and no answering phone calls more than 40 min of waiting

I bought air ticket for my visit home in July..I have to travel from Islamabad to Split Croatia, on 02 July and return flight is on 18 July 2021. I bought Flexible ticket.
Due to my business obligations I have to reschedule my return trip from 18 July to 21 July 2021. I contacted Mytrip for several times. During the phone calls I had to wait for more than 40 min to get agent. In one of the conversations agent told me to refer my change of trip to Qatar airline. I did it and Qatar told me that Mytrip should do reschedule of my trip. Again I tried to call My trip but agent’s are not responding my calls. Just keep me waiting for more time. I am really upset about the situation and additional costs due to my callas from Pakistan to Mytrip in Europe.
If I don’t solve my problem I will put my trip on line as unreliable agency and fraud agency who doesn’t respond to requests of clients. P