Trouble builing .NET Core 2 + Full Framework using 2 Nuget package sources

I have an app that was previously .NET Core 1.1 using .NET v4.6.1 and I was pulling in nuget packages from Nuget.org and also from TeamCity nuget package source. Now when I upgraded it to .NET Core 2...everything fell apart. TeamCity wont build properly, it wont see my TeamCity nuget repository.

What is the best practice to setup a build definition on TeamCity to do the following:

1. Restore packages from Nuget.org (.NET Core and NETFX)
2. Restore packages from TeamCity nuget repository (.NET Core and NETFX)
3. Build the app 
4. Package the app (make nuget package)
5. Publish the nuget package as a build artifact

I can do all steps except for #2 relating to .NET Core projects using full .NET Framework when it tries to restore packages from our TeamCity nuget repository.