Answer by RalfFriedl for NAT a specific IP to go to ppp0 and others to go to eth0 coming from internal wifi interface

Creating an ip rule for that source address is the right approach.

To diagnose such problems, tcpdump is useful.

Can the VPN client itself access the internet through the VPN?

• If yes, you need to add NAT in the VPN client for your address.
• If not, you need to add NAT in the VPN server, and make sure it is not blocked.