'with no internet connection' is the reason of above 'unable to load the service index for source https://api.nuget.org/v3/index.json' error. the app wants to isntall required packages from nuget.org but can't access the nuget api.
If the ADO server is 100% no chance of getting the internet access (even proxy is not an option), you have to use the build-in Azure Artifacts in ADO (that can be used to serve as a package server). You basically manually download required packages from nuget.org and manually upload to the ADO feed, then finally point your build server to use ADO as a nuget server.