Mapping External Subdomain to Multi-Site Subdomain

I am trying to map a test external subdomain to a test multi-site site.

I need to map site1.l2treviewsites.com to site1.clientsite.com.

On the client’s server I have set the CNAME for site1.clientsite.com to site1.l2treviewsites.com

after over 24 hours I am still receiving the “domain name is unavailable to access” error when trying to map