If the WLAN Pi Pro won't charge when connected to a power source (PoE or USB), it is likely it is in a low power protected mode to prevent over discharging the battery. To recover from this state follow the steps below, this will reset the charging IC and should allow the device to recover.
1. Unscrew the 4 screws on the back cover, and carefuly remove the back
2. Remove and reinsert the 18650 battery
Be sure to reinsert the battery in the correct orientation as reversing the battery will damage the circuit boar
3. Reinstall the back cover and screws
4. Connect a power source to PoE or the top USB-C port, verify the device is charging, and let it charge until it has at least 20% charge