Hi Stephan!
Thanks for reaching out to us!
Let me share with you the following article that explains how the QR codes work in Event Tickets Plus > https://theeventscalendar.com/knowledgebase/using-qr-codes-event-tickets-plus/
Unfortunately, there is no offline option to check-in the attendees, as it works by communicating directly with your site.
We are currently working on our own QR code app which will allow you to scan and checking directly to your site without the need of a third party application. We’ll surely post this in our blog once it’s released.
I hope that helps. Let me know if any follow up questions.
Best,
Victor