SUP Office 2019 products

Our Primary server does not have internet access, we manage our SUP by synchronising from a shared SUS server that has internet access and already has the M365\Office 2019 product enabled and can see the updates in the SUS Server catalogue.

We need to sync and patch office 2019, however when we enable the product on our primary server the sync takes hours and fails due "Failed to download file- "http://officecdn.microsoft.com/****" from internet. Using proxyserver - 1. Error = 12029".

How can we get around this issue, when we can not allow the primary server to access OfficeCDN?