There are currently known issues with Apples new Sonoma OS and OpenVPN. Apple are aware of these Firewall/OVPN issues and will release a patch soon. Here's how you can add our VPN via an L2TP setup on your new MacOS.
Click your system settings option and then tap on VPN from the left menu.
Look to the right and click "Add VPN configuration" and select "L2TP over IPSec"
Simply add the following server and shared secret along with your VPN username and password.
Account Name: Enter your VPN username
Password: Enter your VPN password
L2TP Server address: l2tp.mysetup.co.uk
Shared Secret: 953783903
Last step is to tap the "options" button from the left hand menu and select "Send all traffic over VPN connection"
Now you can save and connect.