air travel – TAP Air Portugal changing flight for free is not possible

I booked a flight from Vienna to Lisbon on the 2nd of July, with flight date of 7th of September.
It is a Classic ticket, so basically it permits flight changes with a fee: (75€) + difference.

However in July TAP started a new campaign, as the first change is free for everyone: now they extended this campaign until 31st of August. This was the reason why I bought my ticket, but now things happened and I need to change my flight date from 07th to 17th of September.

Prices are the same, so I think it should be free to change, but their system says I need to pay 75€. I read all their conditions and I am 100%s sure that I should be eligible for a free change.
Also tried to connect customer support in messenger, telephone, even I filled a complaint on their website, but haven’t received any answer after 2weeks of waiting.
Obviously I don’t want to pay 75€ (from this money I could even buy a new ticket), but I don’t know what else options do I have. Cancellation is not possible, I mean I would not get any compensation. (I hope they will cancel it and then I could get a voucher at least.)
What should I do in this situation?
Also, I don’t understand why are they advertising things which are not true?