I'd probably pay the fee, take my flight, then charge it back with a reason like "Fee for not using app, app did not work. Fee advertised as optional was not, in fact, optional for me". Completely true statement.
You would win but then Ryanair would ban you from flying with them in the future.
I don't know about Ryanair, but I've had to reverse credit card transactions due to scams a few times and I've never been banned from other businesses.
Ryanair has a history of banning people who dispute charges.