Demo store not calling custom shipping carriers from private app

[jc33]

I am trying to implement custom shipping rates to a demo store using a private app that I've built.

I have registered the carrier using the endpoint mentioned in the documentation:

https://shopify.dev/api/admin-rest/2022-01/resources/carrierservice#post-carrier-services

If I call the endpoint from: https://shopify.dev/api/admin-rest/2022-01/resources/carrierservice#get-carrier-services afterwards I can see the new carrier.

When I go to the checkout in the demo store, at the shipping stage it doesn't call the callback_url which points to my private app.

In the admin panel shop settings: Settings -> Shipping and delivery -> General shipping rates 

For the zone there is one carrier:

Carrier Name: MyPrivateAppCarrierName

Handling fee: -

Transit time: calculated

Services: -

So it looks like it is associated to the country I want to test. When I go on the demo store's frontend and try to go to the shipping part of the checkout I get the error:

"Your cart has been updated and the items you added can’t be shipped to your address. Remove the items to complete your order."

I have set up a log on my private app when the callback_url is called and nothing get's logged so I don't think the call from the store is being made.

I've tested the custom callback_url that I created in Postman and I can call it and get a response.

I'm not sure why this isn't working. Please can anyone advise a solution or more debugging steps.

Thanks,