Managed DLL - error "the type or namespace could not be found"

Trying to complete the tutorial on using DLLs

but when I create the “Test.cs” script, I get an error at line 3 when I try to build
using DLLTest; "The type or namespace name ‘DLLTest’ could not be found (are you missing a using directive or an assembly reference?)
Mono shows under the project References DLLTest.
I get the same thing whether DLLTest.dll is directly under Assets or in Assets\Plugins.
If I comment out line 3, I get the error at the point where I use the class method from the DLL, same as with the example below.

I also tried the tutorial here (with the DLL built in Mono)

but get the same thing if I add “using RandomNumber;” or I get the error on the line where the function is used if I don’t include a using reference. Again the References show TestManaged.

I’m on Unity Pro 5.4.1f1 and Mono 5.9.6

This looks to me like an application issue, so I tried uninstalling and re-installing Unity but still get the same issue.