Citibank commercial ezlink erp card (04-04-18)

Funny thing happened because i was using the citibank commercial card ezlink erp card. It is an auto top up card. When there is not enough money it will top up by itself. When I pass by the carpark that uses erp it says it is invalid card. I also took out and tap which was also the same. Then after that I went to nex. Then it says insufficient balance. Then shortly it auto topped up. So what got me wondering was how it was able to auto top up. Which I think maybe through the erp payment gateway. The erp payment gateway must somehow be connected to the phone line or the internet. In order to know the card is low on credit and top up the card. So does it only need the connection when it is low then it will do it through the payment processor to topup the card. If not it will not work. That is why when it was low and the payment processor could not connect and top it up it was an invalid card or has insufficient balance?