Trade fair and payment hurdles

Visitors are reported to be facing big problems in the ongoing international trade fair in Delhi. They are not able to make payment through UPI gateway. Every stall owner has a QR code but due to poor signal many users are unable to pay. Finally they are forced to pay by cash. Now the question is at such a big international event, why is the PM's favorite UPI system not effective? Few foreign countries are also participating in the IITF which lasts till November 27.