You need to opt in to enable the designer during installation, it is not enabled by default: https://developercommunity.visualstudio.com/content/problem/4616/linq-to-sql-dbml-designer-in-not-working.html
"Editing .dbml files with a designer surface requires the LINQ to SQL tools which are not installed by default as part of any of the workloads of Visual Studio 2017. It can be installed by selecting the 'LINQ to SQL tools' item under the 'Code Tools' category in the 'Individual Components' tab of the Visual Studio installer."