Unlock Jenkins - instance appears to be offline

I’ve installed Jenkins on VM.
OS: Oracle Linux Server release 7.5
Type: Virtual Server
When I try to Unlock Jenkins, Getting below message,
This Jenkins instance appears to be offline.
For information about installing Jenkins without an internet connection, see the Offline Jenkins Installation Documentation.
You may choose to continue by configuring a proxy or skipping plugin installation.

BTW, Iam behind the VPN.

I tried to validate the proxy using a url. Proxy seems to be fine.

I manage to fix it by editing /var/lib/jenkins/hudson.model.UpdateCenter.xml and change url to use http instead of https. But no luck.

Could you suggest any clue?