This is likely due to a connectivity issue. Either the SIM card you have in your unit is not active, out of data, or placed in the unit incorrectly.
Make sure all 3 cables are connected to the TP-Link/USB modem device. That's 1x grey ethernet cable from the bottom of the payment terminal to the TP-Link, 1x power cable from the USB power board to the TP-Link (only one of the USB ends needs to be plugged in), and 1x aerial cable into one of the side ports of the USB modem.
Make sure you place a working, topped up Optus SIM card in your USB dongle connectivity device, wait for the light to turn a solid blue, and put through a test payment. We can have the test payment refunded, just get in contact with us at email@example.com.
If neither of these work, contact Chargebar at firstname.lastname@example.org and we'll try our best to sort it out for you!