IP Canonicalization Issue

Hi,

I have a website on wordpress (www.toolsqa.com) and a subdomain http://www.onlinestore.toolsqa.com. When I do SEO site checkup on SEOSITECHECKUP.com for IP Canonicalization for my main domain(toolsqa.com), it displays following error:
Your site’s IP 178.79.165.206 does not redirect to your site’s domain name. This could cause duplicate content problems if a search engine indexes your site under both its IP and domain name.

It also provide the solution for above error:
In order to pass this test you must consider using a 301 re-write rule in your .htaccess file so that your site’s IP points to your domain name.
If your site is running on apache server, you could put these lines in your .htaccess after RewriteEngine on line:

RewriteCond %{HTTP_HOST} ^XXX\.XXX\.XXX\.XXX
RewriteRule (.*) http://www.yourdomain.com/$1 [R=301,L]

Note that you must proper format the first line using your IP (replace X characters with proper digits from your IP) and the second line using your domain name.
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Now when I perform the above given solution and change my .htaccess file then everything gets resolved. But now my subdomain is redirecting to http://www.toolsqa.com/onlinestore/. The subdomain still works fine but then I am no longer able to login to my subdomain either through admin panel or from website.

I am using different WP themes for my domain and subdomain. Please look in to this and try to provide a solution. Please use simple words, as I am not a technical person.

Best Regards,
Lakshay Sharma