Kiwi.com Denies Date Change After Charging Extra Fee

I purchased a flight ticket through the Kiwi app for an itinerary with Air Montenegro / Pegasus connections. For the initial purchase, I paid approximately $362. Later, in order to have the right to change my travel date, I paid an additional $93, bringing the total cost of my reservation to $455.

Now, when I try to move my existing ticket to an earlier date using the app—based on the change option I paid for—I receive a message saying “Sorry, we can’t help with this,” and I am unable to complete the change. However, I specifically paid the extra $93 in order to have this flexibility.

When I contacted Kiwi customer service, they told me that because I have already used the outbound flight, no changes can be made and that I need to purchase a new ticket. My complaint is not being addressed. I am not asking for anything extra—I simply want to use the date-change right that I paid for and move my existing ticket to an earlier date.

I am requesting that the travel date change option I paid for be properly applied, that my ticket be moved to the requested date, and that the inconvenience I have experienced be resolved.
My reservation number is 708 408 811.