L-Series External Payment - Can't open page
I've just started working on a new external payment integration, but I'm struggling with the iPad app side of things.
I'm in the staging platform using our own staging account.
I've generated the access and refresh tokens, and I've created the external payment provider.
In the Payments screen of the Restaurant Manager I've added an External Payment button and I see this in the iPad Restaurant app payments screen. However, when I press the external payment button I get the following popup:
I can open the page that I registered during the creation of the external payment provider entry call in safari on the iPad but as far as I can tell the Restaurant app is not accessing the page. I've put logging into the onGet handler, but no entries are being made in the log when the app should be calling it.
I'd be grateful for any help sorting this out.