Solar Panel Not Charging Batteries in Series
12V lead acid batteries typically require around 13.8 volts to charge. So in series your inverter needs to output at least 27.6 volts. Less than that and your batteries may not fully charge. I suspect your UPS may be intended for use with Lithium Ion batteries which have a lower charge voltage requirement. A google search for the UPS you specified did not show any results with specs.