How to trick an Android app to think that it uses mobile internet whilst on Wifi?

I have an Android app which I haven't been to activate because of it requires that during the activation a phone to be a) on mobile data b) with the same sim card that assinged to me by a service of the app.

I have a sim card, but I can't turn on mobile data because firstly I can't top it up here, in other country. Secondly, it'd eat up lots of money, if I could somehow.

Is there a way to trick an Android app to believe that it uses mobile data whilst in relality it's on Wifi? Reverge engeneering would the last resort. Is there an easier way?