If you have trouble connecting to your source database system while working on migration, create a virtual machine in the same subnet of the virtual network with which you set up your DMS instance. In the virtual machine, you should be able to run a connect test, such as using a UDL file to test a connection to SQL Server or downloading Robo 3T to test MongoDB connections. If the connection test succeeds, you shouldn't have an issue with connecting to your source database. If the connection test doesn't succeed, contact your network administrator.