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 mailto:email address removed for privacy reasons, who then told me to go to the forums

What's left that I can try?