On previous versions of vcenter server as vsphere 6.5 you had to enable proxy settings through VAMI in order to apply updates on your infrastructure (for non internet facing subnets)
You can locate your appliance VAMI endpoint from https://ApplianceIP/FQDN:5480
However by following the same procedure download manager on vcenter 6.7 was not able to download updates.
By checking the logs I could determine the
Connecting to vapp-updates.vmware.com failed cannot assign requested address.
It seems that download manager is a different component on newer versions and cannot get internet connection through VAMI. You can see below the location on which configuration files and settings are saved inside the vcsa appliance.
In order to resolve issue I added proxy on download manager as shown on the below screenshot and updates could be downloaded.