In 2015, I wrote the backend for a registration/ticketing/admission system for our (the company I work for) non-profit. They put on yearly galas in a far-off US state. The system utilized QR codes from printed and electronic tickets. The system was used once and royally failed due to connectivity issues. We relied on the venue's wi-fi and had cellular backup. It worked during on-site testing but failed once the crowds formed. Attendee's couldn't load their e-tickets and we couldn't get a response from the servers as tickets were scanned.
Props to you for getting everything working!