Where did you get the domain for WIX?
You will need to follow this article in order to set up your nameservers so they are not managed by WIX.
Change the nameservers here:
Go to this article and copy the record here to add to your DNS records:
Hit Edit pencil button on the right
Paste the A record here:
Edit the original CNAME record and Paste the wwwXX.wixdns.net record here:
Go to https://dnschecker.org/
Put yourdomain.com (A record) and search to see if it's working:
Put www.yourdomain.com (CNAME record) and search to see if it's working:
Put yourdomain.com (NS record) and search to see if it's working:
Go to your website yourdomain.com and see if your website is working.
Now you can set up Mailgun in your domain provider's DNS records following these articles!