I am having the same problem as @William0920 Exchange server 2013 mail certificate works well with ios 13. If ios 14 is updated while the certificate works fine, it will continue to work. But this certificate does not work on a new Iphone/ipad with ios 14 installed. When trying to add a mail account, I get a warning that the certificate is invalid. The resume button does not appear. When adding an account to the outlook for ios app, the continue button appears and the certificate works. I talked to apple support regarding this problem. It says that the secure certificate is mandatory for iOS 14, but does not agree to write this information by mail. Obviously the secure certificate is not mandatory, but iOS 14 does not allow it. the suggestions here won't work.