Same here. I purchased my own domain so I could have more control over my website and now like you, Instagram is saying my catalog doesn't match how I originally set up Instagram shopping. And I don't have the option of changing that domain anywhere. All of my items are the same and everything and everything syncs perfectly on Facebook from my site. It is just my domain that changed. I have contacted Instagram and Facebook support. I'm waiting to hear back.
I think it has to be a FB / IG issue. Because I'm using a Square site and am having this issue. I'm getting no response from FB or IG regarding my two support requests.
It really feels like our Instagram accounts have been put in limbo / no mans land. They aren't disconnected from FB and Instagram, but they aren't connected either. Facebook shows in in my catalog manager / commerce manager that my IG account isn't connected to sell, but then when I go to add it, it shows connected and I'm unable to add it. I've sent them screenshots of all my errors. I'm hoping they help, but I'm not counting on it.