I'm facing a nearly identical problem right now. In my case, I'm a US citizen, the flight is operated in April to the US (LIS-EWR), and paid with a US card. I canceled within 24 hours. The separate TAP refund website wasn't working properly, so I couldn't request the refund within 24 hours. Eventually I got it to work two days later, which I thought would be moot because I cancelled in the 24 hour window. And after calling in, they've said there's nothing they can refund me because I booked it on the "Portuguese website" and so only the fare rules apply. I've filed a complaint with USDOT and just received a short unapologetic response from TAP's US customer service.
I'll reply, copying the USDOT representative, but I'm not optimistic