First, APC UPS are factory set for charging sealed lead acid batteries, and have to be reprogrammed for use with any other type of battery (like yours). So using APC with a battery chemistry/type it is not set for will damage battery. Please do not blame APC for something which is outside it's control!
Second, APC UPS are designed only for occasional, short duration outages to enable safe system shutdown. They are not suitable for daily loadshedding. It is not APC's fault the you are using the UPS in a manner it is not designed or made for.
It is like using diesel fuel in a petrol engine car and expecting it to carry the load of an 18 wheeled truck. Go to the APC website and read their technical papers if you really want to understand why your setup didn't give you the results you wanted.