Turkish not honoring flight change

A few months ago, my partner and I booked a pair of multi-city flights on Turkish Airlines for this July from DUB-IST (with a 5-day stopover) and then IST-HKG on one itinerary. I was curious about changing my DUB-IST leg to a different time (same day) and upon browsing the different options, found that it only cost 3,20 EUR to do a flight change. I have Economy (Q) tickets, so it sounds too good to be true, but sure enough, I received an email for a 3,20 EUR charge and a new itinerary confirmation.Last week, I went to pick seats for my IST-HKG leg of the itinerary and upon trying to check out, it said that one or more of my flights was not confirmed. I called Turkish customer service and they said my ticket was on hold and that I had an outstanding charge of 880 EUR due to the DUB-IST flight leg being changed. Keep in mind that I had never seen this 880 EUR charge when checking out to change my flight (from above), nor did my confirmation email mention anything about being charged an additional 880 EUR.While the flight change would have been convenient (leaving 13:30), I don't mind taking my original flight (leaving 10:50 the same morning). I asked the customer service agent if I could just revert back to my original itinerary (and cancel the hold), and she told me to submit a feedback request, refusing to help me further. She also mentioned that it seems I have no confirmed flights at all with Turkish (even though my credit card has been charged!) I've been waiting almost two weeks to hear back from their "feedback" system and have attempted [unsuccessfully] to speak with two other customer service agents. Is there any recourse that we have to get Turkish Airlines to revert our tickets to the current itinerary since we have paid the full price for those tickets? We don’t want to pay 880 EUR to make the change since it was never stated that that was the total price of the change when we paid 3,20 EUR.   TLDR: We purchased Turkish Airlines tickets and now they are not honoring them unless we pay 880 EUR to finalize a ticket time change that we don’t want.