301 Redirect for primary domain on multisite

Hello.

My original post was here: https://premium.wpmudev.org/forums/topic/new-domain-on-multisite-now-how-to-best-direct-old-domain-to-new-site#post-702588 but have not been able to resolve.

So I have olddomain.com that I would like to point to newdomain.com

newdomain.com is the primary domain on a WP / Subdomain setup

I have tried:
1. Setting up olddomain.com and a PARKED domain in cpanel, and
2. Setting up olddomain.com and a ADDON domain in cpanel,

BOTH times the redirect works fine, but sends me to a registration page : newdomain.com/wp-signup.php?new=olddomain.com.

I have registrations off at the moment, but that shouldn't matter. I'm really just trying to do a simple domain redirect.

Could you help?