Not able to use VPN since upgrading to macOS Monterey

Try disabling Private Relay (System Peferences > Apple ID > iCloud > Private Relay).

macOS Monterey comes with a new feature called Private Relay which routes traffic through an Apple relay so your Mac has different IP address as seen from the WAN side. The aim of this is to protect your privacy. However, VPN services can be confused by the changing IP addresses.

Also see https://support.apple.com/en-us/HT212614

If your VPN (L2TP over IPSec) works properly but you cannot connect to the internet. This is what helped me out. The solution is to add the Google's DNS server to your DNS list.

1. Make sure you have "Send all traffic over VPN connection" selected

2. In addition to the IP that you probably can see here grayed out. Add the following IP 8.8.8.8 (You could provide any other IP to the DNS provider of your choice)

Now VPN should work just fine.