Options for encrypting connection between a reversed proxy and a web server?

I want to set up a reverse proxy that would handle the HTTPS encryption/decryption for my app and only connect in HTTP to my backend servers. That would improve performance as the backed server won't have to worry about encryption, which frees up more resources.

The problem I am facing is that my proxy and backend servers are not on the same network, which introduces security concerns. I don't want the communication between the proxy and the backend server to be transmitted in plain text. What options do I have to encrypt data transmission between the proxy and backend server without compromising the performance?