How to install Office LTSC Professional Plus 2024 with no internet?

Question

My company is a defense contractor with classified (no internet) and unclassified (with internet) networks. Some of the computers on the classified network require the Office suite. In the past we've been able to install the software using KMS or using the offline Office installation.

Now we are trying to install Office LTSC Professional Plus 2024 which downloads a click-to-run setup file instead of an ISO like some older versions of Office. We can transfer the setup file from the unclassified network to the classified network, and the software installs successfully on the unclassified network. However, it is asking for a product key, and there is no key found on the Microsoft Business Center where we downloaded the setup file.

The users in the classified area all have M365 subscriptions that include Office, but their credentials are not recognized because there is no internet access. How do we proceed? Thanks.