Nuget not searching in custom package sources

I have added 2 custom package sources. If I search in nuget package manager, I find my package. If I install the package it fails with the following error:

Restoring packages for .NETStandard,Version=v2.1...  GET https://api.nuget.org/v3-flatcontainer/mypackage.test/index.json  NotFound https://api.nuget.org/v3-flatcontainer/mypackage.test/index.json 125msResolving conflicts for .NETStandard,Version=v2.1...Unable to find package mypackage.test. No packages exist with this id in source(s): nuget.orgPackage restore failed. Rolling back package changes for 'FormsApp1'.

In the logging I see that package manager only searches in nuget.org and not in my custom sources.

I am using VS for mac 17.4.2, Nuget 6.4.0.117

Questions:

1. How do I tell package manager to also search in my custom sources?
2. How do I install a specific version of nuget on Mac? I would like to try version 6.3.1.1.