The free version of ProtonVPN, the only one that works with OpenVPN on SFOS is the one relying on UDP and it is limited to the IPv4 traffic only - therefore it leaks all the IPv6 traffic.
In your SIM registration with your network operator change Dual to IP and in the advanced options of the OpenVPN config select Disable IPv6: yes.
- ipleak.net is your friend
Finally, your smartphone will continue to leak some data like DNS queries and WebRTC. With Firefox (and SFOS default native browser) you are able to disable that leak but with Chrome/ium you need to install an extension that can read all the website that you visit unless you did not disable that feature.
About DNS, take a look at the Quick Start Guide (here) and choose the solution you prefer.