Flight Network has the worst customer service

The worst and most frustrating experience i have had with a flight agency. I booked tickets via FlightNetwork 2 months ago and had to change the dates about 3 weeks before the departure date due to covid-19 restrictions, I have been trying to do a rebooking or get a refund since then. Every time I call their support call centre, I'm onhold for more than 1hour on average and hardly get through to an agent. Out of a minimum 20 calls, I have only gotten through to them twice and both times the calls were not fruitful. I have been given a deadline to rebook by the 05th of Aug 2021 but I can only do the rebooking via a call, problem is when i do call i don't get through. Not sure what exactly it is that I'm supposed to do at this point! Do not use this agency, worst customer service!