Symptoms:

User receiving the error message: "Safari cannot open the page because your [iPhone/iPad] is not connected to the internet" when attempting to access a VPN gated resource.
(Such as when attempting to load the OKTA page while authenticating their PARTNERS(OAUTH) email account.)

Causes and Resolutions:

User may not actually have network access. 
(This can be confirmed by having the user access non-Mass General Brigham sites.)

If the issue affects only Mass General Brigham VPN-gated sites then the issue may be related to the user's VPN connection. 
(This can be confirmed by having the user attempt to login to other VPN gated sites such as myprofile.mgb.org)

For users enrolled in MobileIron:

Verify the Mass General Brigham VPN configurations, following KB0038305

If the issue persists after verifying the VPN configurations, re-enroll the device per KB0039599

Remove old MDM profile restored from backup when getting new phone or restore phone following Remove MDM profile

For users enrolled in Intune:

If they have not installed and configured Cisco Secure on their device, see KB0041849

If Cisco Secure is installed and configured on their device, then the ticket may need to be escalated.

Escalation:

If escalation is needed for Cisco Secure, contact the Digital Service Desk to have a ticket opened with Network Security - mgb

If escalation is needed for MobileIron's VPN configs, contact the Digital Service Desk to have a ticket opened with ETM - Operations MGB