Wordpress Multisite Subdomain

I have Wordpress currently setup with subdomain multisite. I have
multiple domains that are aliases for a primary domain. With the rules
I have in place, going to one of the secondary domains results in a hard
redirect to my primary. Example:

Enter http://secondarydomain.com in the browser.
Get redirected to http://primarydomain.com.

Is this the only way I can have it work? My preference would be to have
the server name remain the “secondary” name as entered - so it appears
to be multiple discreet sites.