IPad Printing

You could try saving the email you want to print as a PDF, and then loading the PDF into the print app normally.

Tap on the twisted arrow at the bottom of the email message, scroll down to the Print option (you aren't actually going to print it from there), zom into the preview by reverse pinching the preview, then tap on the share icon (arrow coming out of a box), then tap on "Save to Files" or to the Print App directly if its an option there.

I don’t have that app, but when I used another third party app to print emails, I needed to access my email through the app. So if the brother app has the ability for you to enter your email info, or use the app’s browser to access your webmail (does the brother app have an internet browser? If so, let’s say you use yahoo for your mail, use that browser to go to yahoo.com, open the mail then try to print it

You may have some success in using a separate third-party printing App such as Readdle Printer Pro.

The Readdle App is particularly good at providing a print-solution on iPadOS/iOS devices where it doesn’t otherwise exist. There is also a “helper” component that can be installed on your PC that provider Print Server functionality to non-AirPrint printers.

I hope this information is helpful in finding a workable solution.