Here's an overview of how I have things setup:
All of our domains are registerd with GoDaddy and our web hosting is with Siteground.
I have the main hosting account domain (domain.com) currently pointing to a Shopify store.
I then have an add-on domain setup (domain.net) which I've setup as the main WordPress Multisite Network site.
I've also got our German domain setup as an add-on domain (domain.de).
Both the .net and .de domains have their DNS pointed to the Siteground hosting.
I've setup a subfolder network site for the German site (domain.net/de) and this works great.
However, I've tried to then set this subfolder site up so that it works from the domain (domain.de) but I can't seem to get this working. Either when I have the add-on domain setup to have the same 'home' folder as the .net domain, or if parking the .de domain with a redirect to the .net site.
Our main site is running WordPress 4.9.4 so I don't believe we need a plugin such as 'WordPress MU Domain Mapping' to get this working (although I have tried that method too).
With the .de add-on domain set in the hosting to point to the .net domains home folder, the closest I get to it working is the Siteground 404 page.
I'm pretty stuck on this now so any help would be greatly appreciated. Also, if you need any clarification on anything or have any questions, please do ask.
Hope to hear from some of you soon!