As of 20191003, you should be using PHP 7.1 and above for your WordPress websites.
However, some older plugins can cause your WordPress Dashboard or wp-login.php page to fail with an infinite redirect loop.
To diagnose, set ‘WP_DEBUG’ to true in your wp-config.php file, then attempt to login or visit your WordPress dashboard.
If it fails, head to your cPanel/WHM to change your PHP version for your website to 5.5, 5.6, 7.0, 7.2 and the likes.
While switching, you should be able to see error messages to help you diagnose the plugins that is causing the problem.