When you click on the offer, it takes you to "Your choice" page and there it states "Promotion 19.- instead of 49.- " and a little info icon which displays:

"Details about the promotion
For the next 24 months, you will benefit from a monthly discount on your Mobile internet subscription."

I did not proceed further but assume the contract spells out some more conditions on the 24 months duration.

Clearly, it is not described well in the initial product page