dschungelkompass.ch lists all "special offers" available in the telecom branch:
https://www.dschungelkompass.ch/news/aktionen/

Also your offer is listed there, clearly with a 24 months minimum duration.
https://www.dschungelkompass.ch/news...statt-49.xhtml

May be lightly deceptive with an entry page stating no minimum duration and a change on the next step/page, but as @jjake writes, it was stated also in that page, while still hidden behind the i icon.

Don't think you will find any valid legal grounds to fight this, but there might be goodwill on sunrise's side such as allowing you to step back from the contract. But I doubt ...