Domain name shows on home page but host url shows ons all others

I have searched for help on this but what I’ve found isn’t working for me.

I built a wp site for a friend and then we set up his domain name to point to the wp install (in a subdomain of my site).

When you type in his domain (https://amertrkins.com) it goes to the front page of the site. But if you go anywhere else, you see my hosting url (https://mamaestes.com/ATI/new/blahblahblah).

I’ve tried to reset the site and home urls in the settings – it takes the whole site down. Same thing when I reset them in my MySQL dbase.

There has to be an easy way to do this – can someone tell me/point me to the right forum post???

Thanks!