Hi Abby!
Thanks for reaching out and thanks for choosing one of our Platinum WordPress Hosting partners. Sorry to hear about the login problems after changing the domain name on your website and our team will do everything we can to help!
Is your website running any caching or security plugin? If so then it is possible that those plugins are holding onto some of the old domain information that might be interfering with login attempts on your mobile devices.
Another potential cause could be the browsing cache for your iPhone and iPad. If you haven’t already done so, try clearing the browsing cache on your mobile and tablet devices to see if that allows you to get the login working properly.
Explicitly defining the Cookie Domain for your website in your website’s wp-config.php might also help fix the problem you are seeing. To define the Cookie Domain, open up the wp-config.php for your website via FTP or with your web server’s File Manager and add the following code just before the line /* That’s all, stop editing!…*/ in that file.
define('COOKIE_DOMAIN', $_SERVER['HTTP_HOST'] );
I hope that one of those solutions helps you get your login functioning as expected on your mobile and tablet. Please let us know if you have any additional questions for us, we are always happy to help!