There are multiple similar services to ZeroTier. You could also set up your own VPN. You could also set up a tunnel via SSH (ssh from the Pi to the server and add a tunnelled port to the ssh config.)
Whatever you do, remember that your Pi is likely to be less well protected than your server. So a point-to-point VPN into your server might be a pretty dreadful idea. Consider having a DMZ to coordinate incoming IoT traffic with additional inner protection to your inner network and servers.