Question about website google indexing!

GUYS, PLEASE HELP ME. I didn't sleep last night trying to find a solution but I'm still stuck!

TL;DR – An addon domain is apparent in google search instead of the main domain.

Hey guys, I'm freaking out a bit and really hope that I can get some help here!

I made a WordPress website for someone on his domain and I was doing some tests on an addon domain that I added to this domain.

After finally finishing with some SEO touches I added the domain URL to Google Search Console using the domain prefix option and pasted the HTML file google have me to the public HTML folder, the site was failed to index first (yesterday), but now (today) it is shown indexed on Google console.

When I search for the company name or just search "site:xyz.com" on Google I still get results for the addon domain, both today and yesterday, and don't see the main domain at all in search!!! Even the addon website images were added to google images! How!? I didn't even submit it to indexing. I even went as far as deleting the addon domain and it's WordPress installation, still noting!

I have no idea what to do now in order to remove the addon domain from search and replace it with the actual domain.