Oh! And one last note – almost forgot to mention it…
It is not very likely, in my opinion, but one possible cause of all your issues here could be your site’s .htaccess file. This article explains what this is and how it should look for various configurations of WordPress, just in case you’re not familiar with this → https://codex.wordpress.org/htaccess
It may be worthwhile for you to send a support request your Web Hosting provider and see if they can look at your site’s .htaccess file, and see if there’s anything at play there that could lead to the sorts of issues we’ve been addressing in this thread…
Just a possibility – cheers William!