Mobile PDQs need internet connectivity of some form to operate – some are designed to use Wi-Fi, some use a wired network connection but most use the mobile phone network to connect. The connection to the mobile phone network in most PDQ terminals is GPRS rather than 3G or 4G. On a busy event site the mobile PDQ has to contend with all the other devices trying to connect to the mobile network which often struggles with demand so PDQ transactions fail. GPRS terminals are particularly prone to problems as it is an old and slow technology which does not perform well when under heavy load.
The best approach at events is to use a Wi-Fi or wired IP terminal as these avoid the mobile networks, connecting to a dedicated network for payments instead. Many events offer a rental service or suggest a recommended supplier.