Tutanota has a section on how to set custom domains up but I did not find it very easy to follow as the information was not specific to Dynanot.
Many registrars use different interfaces or even naming conventions, so it took a bit of trial and errors to make it work.
As you can see from the official Reddit forum for Tutanota, many users find custom domains difficult to set up.
So for anyone who is trying to set up their Tutanota custom domain, here is a copy of a working configuration for a domain registered with Dynadot.
Please let me know if this was helpful.