Mapping domain to Multisite Subdomain

I think this is a common issue as i’ve found many others with the same problem.

I’ve created a multisite and want, auxilium.jamesg.duckdns.org to be independent on auxilum.scot while still being in the network.

It currently works and the domain will bring me to the site but i have a couple of issues.

1) When changing between pages, the URL doesn’t change
2) I can’t access the /wp-admin through the new domain, only the old one
3) The mobile site doesnt work and on mobile devices it continues to use the desktop version

I’d really like to get this URL problem fixed, Thanks in advance!