On the mobile app, go to the booking page, click “help centre” at the bottom, scroll to the bottom of that page and select the “Still need help option”, then you should be able to select the booking it’s about and get a phone number.

You can then keep ringing that number without going back through the app. First person you speak to will likely be the first line support and you will need to be transferred to their resolutions team, you only seem to be able to escalate 1 level before being told the decision is final.

I would record everything because you will get given contradictory statements and the transcriptions if you request them are inaccurate.