You need a VPN that can split tunnel by ip via CLI (although I think it's also possible to set it up in an ovpn file, but I haven't tried it). The only one I've found that can do this natively is proton, specifically the python community version.
I don't know how this next part works if you use something that isn't tailscale, but if you do then just set proton's split tunneling for 100.64.0.0/10
Then, still on this machine, advertise the exit node from tailscale (you also have to allow it from your tailscale admin console). Connect to it from your phone, making sure to use the server as an exit node, and head over to ip.me to see if it's working