Seems like offerwall of mobilerewards.io have heard of them few times before but haven't tried or paid by them. But would they allow to use VPN to do offers? if they detect it they may not pay or banned even your whitelabel service account OP, better to check about using VPN in their TOS.
This is my first time I am using .
Suggest me how to promote offers ?
I haven't used their wall before, but i think if you can make a mobile version website with good domain dedicated to the offerwall only and promote them you will get good traffic. And if you have budget a simple mobile app which just visit your site inside frame of the app, building this could be cheaper.
I haven't done similar site before but as i am also developer i know developing this all will not cost you more than 200$ on starting. Good luck, but better you try promoting CPA offers for web rather than this mobile apps, this pays really low.