Multisite primary site, when accessed without "www", not located

I have a site running at http://www.contexthq.com

It is powered by WordPress Multisite, which is set in subdomain mode, not folder mode. New-site registrations are switched off, since I am not running a public network, I am using it to power a couple of my own sites.

There are three sites in total:
1. The one at the above URL, which is the primary one, accessible via http://www.contexthq.com
2. braincloud.contexthq.com, private
3. robert.contexthq.com, private

The problem is, when I access the primary site via a link like http://contexthq.com/about/ (no "www" subdomain), it is not located and WordPress throws the Multisite error message: "Registration has been disabled."

How can I ensure that, if "www" is omitted from URLs, the page/site still resolves to the "www.contexthq.com" equivalent?

FYI, I am using a VPS package. In my CPanel, there is already a subdomain set for *.contexthq.com, with a "document root" set at "public_html" and no redirection set.
In my site's zone settings, I have A records for both contexthq.com and *contexthq.com, pointing to the site's IP address. There is a CNAME for http://www.contexthq.com.

My .htaccess as it stands is...

# BEGIN WordPress
<IfModule mod_rewrite.c>

RewriteEngine On
RewriteBase /
RewriteRule ^index\.php$ - [L]

# add a trailing slash to /wp-admin
RewriteRule ^wp-admin$ wp-admin/ [R=301,L]

RewriteCond %{REQUEST_FILENAME} -f [OR]
RewriteCond %{REQUEST_FILENAME} -d
RewriteRule ^ - [L]
RewriteRule ^(wp-(content|admin|includes).*) $1 [L]
RewriteRule ^(.*\.php)$ $1 [L]
RewriteRule . index.php [L]
</IfModule>

# END WordPress

# Modified for Multisite https://www.namecheap.com/support/knowledgebase/article.aspx/9191/29/how-to-create-a-wildcard-subdomain-in-cpanel
# Previous here:
# RewriteEngine On
# RewriteBase /
# RewriteRule ^index\.php$ - [L]
# RewriteCond %{REQUEST_FILENAME} !-f
# RewriteCond %{REQUEST_FILENAME} !-d
# RewriteRule . /index.php [L]

Is my solution to be found in WordPress settings, CPanel redirect, .htaccess, domain mapping add-ons or something else?

Thanks.