You could try the following macro which will display the classic Spelling and Grammar dialog box:
Sub DisplayClassicSpellCheckDialog()
'Macro created by MVP Stefan Blom, May 2018
On Error Resume Next
Dialogs(wdDialogToolsSpellingAndGrammar).Show
End Sub
Add the macro to the Normal template and attach a keyboard shortcut to it. See also http://www.gmayor.com/installing_macro.htm.