I am unable to get TestFlight from Toodeledo on my iPhone 15

I use an app called Toodledo on my iPhone. They are beta testing a new version on TestFlight; when I bought a new iPhone 15 the app wouldn't work. I deleted it and downloaded the legacy app but now can't access the beta version any more. I get a message on TestFlight "Could not install Toodledo (Next Generation) App".

Toodledo can't help. How do I get this to work? Do I need a new "Redeem" code on TestFlight and, if so, how do I get it? Or is there something else I can do? I am currently using the beta version of the app on my Mac and iPad but the legacy app on my iPhone, which is not ideal.

[Re-Titled by Moderator]