Can't verify in-game purchase via mobile app

Wondered if anyone on here could help, I've spent half an hour getting increasingly angry with the Meta "customer support".

My son has a Quest 3S.  He wants to buy a bundle/add-on for one of his games.

He starts the process on his headset.  It then says in the game "continue in the mobile app and follow the instructions to complete the purchase".

I open the mobile app.  There is a notifcation saying "tap to verify purchase".  When I tap it, a white screen pops up and I receive a text message from my bank with a pin number to enter to approve the purchase.  However, the relevant page on the app with the input box for the code will not load.  There is no way for me to enter the pin number to complete the purchase.

I tried going on the meta horizon store website via my PC and searching for the add-on via the website - this has got me around this issue the last time it happened, but this time, I can't find the relevant item on the webstore.

I tried to contact Meta support via their AI troubleshooter, but it keeps telling me that I don't have a device associated with my account and won't let me go any further.  It does give me the option to enter the serial number of my headset, but when I do this, it says "support is not available for this device".  I then tried to contact support via the mobile app instead, but this time it loads a page saying that help isn't available due to a technical issue.

I've never actually known such awful customer support, I don't think words exist to express how frustrating this is that I can't complete a simple purchase.  It's as if Meta don't actually want me to buy anything.  They certainly don't want to help me.