Hi Lewis,
>
> Are you serving over HTTPS?
>
Yes, all content over HTTPS

> Can you create an MCVE (minimal, complete, and verifiable example) for
> a
> ".txt" file over HTTP? For a ".txt" file over HTTPS? For a ".ivp"
> file
> over HTTP? For a ".ivp" file over HTTPS?
>
Sorry, I don't know how to create this.
> In your Pastebin access log, I see occurrences of "$1" in your
> requests.
> Is that intentional, or is that a bug?
>
Yes, that is intentional.

> What Content-Type header field are you sending in your response to the
> request for a resource ending in ".ivp"?
>
They are text/xml;charset=UTF-8 and application/json.

> Can you try adding a user name and password for the website on the
> iPad
> under Settings > Passwords & Accounts > Website & App Passwords?
>
Yes, I already do this, but the browsers still promp password
> What happens if you try Firefox and Google Chrome on the iPad? Do
> they
> work?
>
neither of them work in Ipad/Iphone.