I have spent far too much time trying to resolve, what is, a fundamental feature. Perplexed as to why spell checker will not work.
Onwards. I need to invest my time in other projects. I have found something else to serve my mail needs. I am so glad I didn’t immediately pay for MailSrping. It ticked all my boxes too.
I hope you get it resolved one day – seems to have been a long standing issue (2 years) from what I can see.
Best of luck