VS Not Recognizing Project Classes for Intellisense

Hello,

I recently installed Version 2019.1.4f1 with the Visual Studio 2017 from the bundle, which as of the upgrade from 2018 does not seem to recognize any of the other classes in the project with Intellisense to show them in blue highlighting, including basic Unity classes e. g. Transform, GameObject and MonoBehaviour.

Also, very often, not always, it will re-open another Visual Studio instance when jumping to a source file that it already has opened in the first.

Finding a number of people who reported a similar difficulty with upgrades to 2017 and 2018, I have tried all of the following no no avail:

-Closing and restarting both VS and Unity.

-Deleting the .sln file and having Unity re-create it.

-Seeing that the "source" editor in "Preferences" is "Visual Studio 2017 (Community)", also for kicks setting it to "default" and toggling "Editor Attaching" on and off, all with no difference in effect.

-Bringing up individual files in the Solution Explorer in an attempt to tell it to add them individually.

The top line says what translates roughly to "Number in project: 0", below it all three say "Unavailable" and below them, "void of files".

To get to the "Show All files" button, one had to first go to the "File Explorer". Right-clicking .cs files there does not present the "add to solution" option, indicating perhaps that it may already recognize them as part of the solution, as they appear there even without the "Show All Files" option.

By the way, I removed an old project backup that was within the folder, in case they conflicted, deleted the SLN again, and it's still seeing the old ones when it refreshes the SLN's; they just produce an error when you select them from the drop-down arrow to the right of the fourth icon above that looks like a folder.

Anything else to check to get it on board?

Thanks.