I'm not sure I understand it all. However, I did manage to get the DKIM Signature to appear (correctly). I am not sure whether it needed isSMTP() but I added that line. It still says dkim=fail (signature syntax error), but maybe they're not going to clients' spam. I'm still trying to resolve that. But, now I'm more worried about the PHP Mail function I used for other auto emails sent. Maybe they are all going to clients' spam folders so many are never been read. Do you think it's a bad idea to use PHP Mail function? It's usually for small reminders to clients, not any bulk emails.
(I did try a different tester but not the one you suggested yet)