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.
https://support.wix.com/en/article/dns-records-needed-to-connect-your-domain-to-wix
Change the nameservers here:
Go to this article and copy the record here to add to your DNS records:
https://support.wix.com/en/article/dns-records-needed-to-connect-your-domain-to-wix
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!
Step-by-step guide to set up Mailgun
How to Set Up a Dedicated Sending Domain (LC Email)