For the DOI, do you have a Campaign?
So: you have a form, then the contact starts the campaign and you send an email with a link.
The execution of the campaign should work independently of the IP of the contact, as Mautic adds the tracking code to the link and the click will be recognized. This does not work “zuverlässig” by you?
Can you add a screenshot of your campaign?
On the other hand: if the same contact visits any of your pages from another IP, as I know, Mautic will add his visits to an “unknown” contact and won’t link his visits to the known contact. I suppose this is not your question, right?