The major ISP's all seem to have special requirements when it comes to setting up DNS for MailGun. GoDaddy and 1&1 IONOS both require details which are not covered in your instructions and not commonly known by their first line support.

It took me over a week to get DNS properly configured with these two ISPs. How about either (1) getting instructions for major ISPs or (2) contacting major ISPs with your requirements or (3) giving a list of common pitfalls?