How can I install a user CA certificate from a .pem file?

I have a .pem file and -key.pem. I'm on a Pixel 4a with Android 13, and when I go to Settings -> Security -> Advanced Settings -> Encryption -> Install certificate -> CA certificate, I can select one file. When I select the .pem file it just goes back to the Install certificate screen, and when I choose the -key.pem file it says that I need a private key to install it. I don't know much about SSL certificates so help would be very much appreciated :) ---------- Edit: I am using mkcert to generate a certificate for a certain IP address in my network. I am using KDE connect to send the files to my phone.