I built a VPN application for iOS and Android (still working on it) that uses user's DigitalOcean, AWS, or GCP to log in via OAuth2 or API token and create a VPN server so that only the user can connect and use.

However, I still can think of any monetization except Ads (which I don't think will help because the VPN server also include ad-blocker). The only monetization might be donation. But if anyone has any idea I would be glad to listen to.

Btw, the link to side-project: https://www.zudvpn.com GitHub: https://github.com/zudvpn/ZudVPN