The Package Manager version of TextMesh Pro should be the only version used with Unity 2018.1 and up.
The older versions of TextMesh Pro from the Asset Store are no longer getting updated.
With regards to addressing the Namespace issue, are you using Assembly definition (.asmdef) for your own code? If so, you will need to add a reference to the com.unity.textmeshpro assemblies in your own .asmdefs. There are a few posts already explaining this process on the forum and in the sticky thread here .
If you are not using Assembly definitions, then you have run into a known issue potential related to Assembly Definitions and project solution synchronizer. We are still trying to reproduce this issue on our end and still need bug reports submitted with the project exhibiting the issue.
In terms of the issue itself, there are two potential ways to get around the issue. The first is to update Visual Studio or which ever code editor you are using to the latest release. Most of the time this resolves the namespace issue.
Another potential solution that some users have posted about is to change the External Code Editor in the Unity Preferences to some other Editor and then back to which ever one you were using.
Let me know if these two potential solution resolves the issue for you. If not please submit a bug report with the project so that we can reproduce this behavior and fix it.