VPN users cannot sync files, but can access outlook email

I have a few users who recently started experiencing this issue. While connected to the VPN, they can send/receive their email through Outlook fine, but when they go to sync their files, they are unable to. When I go to ping our server from their machine, it resolves to some unknown address, not the internal address it should receive while on our VPN. However, when I ping our email server, it works just fine.

As a workaround, We have changed their DNS servers to a manual address that allows them to access and sync with our servers.

Any help would be greatly appreciated. Thanks in advance!