But if you only got that IP and a MAC-Address - how do you know which tenant is supposed to adopt the device?

We support two approaches, you can either pre-register MAC-Addresses or you can add source IP's to assist with that mapping. There is more information in our docs about this: https://tamarack.cloud/docs/migration

that's the key information to understand the whole thing.

it would be great if that could be in the article in the first place. (I'm assuming you are the author)

Good call, I should have made that more clear. Article updated!