How to active Windows IoT Enterprise LTSC2021 via phone

Dear Community,

how can I activate Windows IoT Enterprise LTSC2021 via phone. I MUST activate via phone since the computers are installed in a facility without internet connection. What I have already tried that DID NOT work

1. The activation dialogue does not show the "phone option" as seen in this video: https://www.youtube.com/watch?v=6RTu0_uWjMM
2. Run "SLUI 4" from the command line with admin privilleges. It just returns without doing anything
3. Rebuild some internal databases as described here before step 2: https://answers.microsoft.com/en-us/windows/forum/all/activate-windows-by-phone-not-there-in-settings/34b72b93-563f-463c-8614-4675cfd0143f
4. Contacted the seller of the license (arrow), they referred me to email address removed for privacy reasons, who then told me to go to the forums

What's left that I can try?