I'm having the same issue when using MotionPro Plus after I just upgraded to Big Sur. Under Catalina it worked just fine.
Now, when I enable Secure Tunnel I cannot resolve any internet DNS names anymore. I can connect to any internet IP, it just doesn't resolve any DNS. Any ping will just hang after I connect over VPN using MotionPro Plus.
This does not have anything to do with how the VPN is set up. I'm using the exact same configuration as under Catalina.
Wish I didn't upgrade now.