Hello!! Ive been struggling with this as well and I finally fixed it!!!
You will need to go to your shopify Admin where you can edit “domains”.
You’ll want to swap your primary domain with your secondary. Thus the new domain will be your secondary and your primary domain will be your old shopify domain.
It took about a day for the change to go into effect but now all of my items are available on my instagram shop and I can tag them in posts.
Hope this helps!
Ok, I fixed it properly! I literally tried everything and now it works perfectly, with my new domain and all is great. I spoke to Facebook, who said it's shopify's problem, and shopify said it was facebook etc. Eventually someone at facebook suggested this and it actually worked.
The problem is that when I/you signed up for instagram shopping your domain was set, and if you then changed your domain, it doesn't work as you can only have one verified domain on Instagram (and the ONLY way to change is to delete everything and start again).
You need to basically delete everything and re-apply for instagram shopping with your new domain. It's a huge pain but it's worth it and actually works.
You need to go on your facebook business manager, delete your Shopify catalogue/all products. Delete your facebook shop! Delete everything shop related and change to a personal instagram account. Delete the facebook/instagram channel from your Shopify admin. Go on instagram and settings disconnect the facebook page associated with your account. Go on facebook page settings and remove instagram from your page.
Then I waited a few days on a personal account. The 'shop' tab was still visible on instagram but there were no products there and no catalogue connected. After a few days I went back on Instagram, changed to a business account and then went on settings and 'shopping', and applied for Instagram shopping. Then I set it up on the phone, and I set up the facebook channel on shopify and updated catalogue etc. I can't remember if I had to set up the catalogue before applying again for shopping but I don't think so. Either way it should work. the main thing is to get to the point you can re-apply for instagram shopping and it will ask for the primary domain PUT THE RIGHT ONE. Mine was www.waw.group.
Good luck, I was stuck helplessly for weeks.