The PAX terminal will only work if plugged into a power source

There can be 3 sources to this issue:

The pax devices come from manufacturing with a small piece of plastic paper between the battery and its connector. This plastic piece prevents the battery to charge if the terminal is plugged into a power source. It needs to be removed upon initial use for the terminal to be able to charge correctly.
If it is correctly removed and the terminal shuts down as soon as it is unplugged then the battery might need to be replaced. In this case ask the merchant to run a battery test https://handpoint.atlassian.net/wiki/spaces/PD/pages/3860070401 if the test fails the battery needs to be replaced.
Sometimes the merchant will say that the the terminal only works when plugged but actually do not refer to the battery. What they mean is that he transactions initiated from the POS do not start a transaction on the terminal when it is not plugged into power at all times, it goes into sleep mode. When the terminal does not wake up automatically from sleep mode when a transaction is sent to the terminal it is probably because the merchant clicked "no" to the battery optimizations question upon initial installation of the Payments app. Without those permissions enabled the terminal will not be able to wake up from sleep mode when receiving a transaction over the cloud. They can manually enable this through the Android settings : Apps --> Payments --> Battery Optimization --> DO NOT Optimize (otherwise the terminal will shut down its internet connection when sleeping)