Double-click on the .cer file (ex: HostingInside VPN CA.cer). Keychain Access will pop up with a dialog that says “Keychain Access is trying to modify the system keychain. Enter your password to allow this.” then enter Your Password.
In the Keychain options choose login and click Add
Go to login -> Certificates tab and double-click the newly imported VPN certificate.
Open Trust section and change IP Security (IPsec) value to Always Trust
Close the small window and enter your password. Your settings will automatically save after entering the password. After that, you will see your certificate like this.
Close the Keychain Access window and open System Preferences. Go to Network -> Open the 3 dots (...) -> Add VPN Configuration -> IKEv2
In the Display name field, You can enter it with the name You like (ex: My VPN). Enter both Server and Remote ID fields with the VPN server's domain name. On the User authentication, select Username then enter your VPN Username and Password
Click on Create and a new VPN option will show up. Toggle the switch button and the VPN is ready to use