The specifications that you gave us are still somewhat vague, anyway the answer was already provided above, whatever the data you add to the thread. For a highest autonomy, you need simply add more A.h ( Ampere-hour ) capacity to the battery bank, being it inside if there is enough space available, or externally to the equipment.
However, if you're dealing with a commercial cheap UPS, need to be aware the it is a common practice in that indutry to define the inverter transformer undersized to its nominal output wattage, if the autonomy is not sufficient to heat it significantly. It happens because in general the heating at the core happens at a very small rate. It means that if the relation among the power and core sectional area of transformer do not satisfy: \[{ A}_{e }\]≥\[\sqrt{ P }\], adding more batteries or increasing their A.h capacities would overheat the equipment.