I have an TN-1500 inverter. Why is the LED indication of AC IN not lit after mains input has been applied?

According to the mains voltage of different countries, the output of TN-1500 inverter 110VAC version can be altered to 100/110/115/120VAC. In the same way, TN-1500 inverter 220VAC version can be changed to 200/220/230/240VAC as well. When the inverter is set in UPS mode and the mains voltage fluctuates over ?5% of the set AC output voltage, the inverter will shift its power source from the city power to battery to remain the accuracy of the AC output Voltage. Meanwhile, the AC IN indicator on the front panel of the inverter will be turned off.