Unable to add eSIM on used iPhone 12

I bought a used iPhone 12, which I have now set up just like my old one, same model. The problem is that I can’t add my eSIM to the “new” phone. I don’t see the option “Add Cellular Plan” under “Cellular” (or “Mobile Data”). My phone number is listed, but it says “No SIM.” I deleted the eSIM on my old phone, but I can’t find any way to select and add a new eSIM. When I scan the eSIM QR code with my old phone, I can proceed, but not with the new one…

This is translated from Swedish to English, so sorry if the “phone terminology” isn’t correct, but I hope it’s understandable anyway.

[Re-Titled by Moderator]

Original Title: esim