Why am I getting a 'DNS leak' warning with Proton VPN?

Yes, but you may need to try specific servers optimized for bypassing restrictions.

Open the app, go to settings, and toggle the split tunneling option to select which apps you want to tunnel through the VPN. The steps to enable split tunneling in Android and Windows is same. However in Linux, you have to open the configuration menu with protonvpn configure, then select Split Tunneling with '6' and confirm with 'y.'

Use your Proton VPN account information to log in after downloading and installing the Proton VPN app on each device you wish to secure. If you have a Proton VPN Plus plan, you can connect up to 10 devices at once using a single account.

Proton VPN provides a money-back guarantee for 30 days. Unless you purchased with cash or a bank transfer, you can receive a complete refund if you cancel within this time frame. Depending on how long your plan was in effect and how long you utilised the service before cancelling, you might also qualify for a prorated refund.
Go to your account settings on the Proton website, find the subscription section, and follow the cancellation prompts.

If Proton VPN is not connecting, then:

• Check your internet connection, first.
• Try a different server.
• Confirm your login information.
• Make sure your firewall and antivirus settings aren't preventing the connection. 
• Restart the application.
• Update Proton VPN to the most recent version.

Get in touch with Proton VPN support for more help, if required.