Google tag not firing on booking?

Hi, I have the cliniko booking form integrated into my website via en embedded iframe. I have set up GTM in the clinko account settings (container ID is entered) and I have a custom tag and trigger set up in GTM based on the /bookings/confirmation URL appearing in a page view. I also have all page views configured as a tag & trigger flowing through to GA.

GA is showing pageviews and other non-iframe (i.e. same domain) tag/trigger combos from my website so I know the GTM/GA setup itself is working.

The issue I have is that the tag/trigger that is looking for the bookings/confirmation url works (trigger fires and tag is hit) when “prevent cross site tracking” is disabled in my browser (which coincidentally is required in the GTM preview/debug mode) and an event generated in GA.

However, when the “prevent cross site tracking” option is enabled, the custom trigger/tag does not fire and no event appears in GA…

No matter what I do, I can’t get a goal in GA to register using the pageview tag/trigger and the goal destination as bookings/confirmation.

I’ve even gone as far as to allow for cross-domain linking but no luck.

Any suggestions what I doing am wrong?