Stay FAR away from PSS

Spent a small fortune with UPS/Inverter and AGM batteries. Worked fine at first but then had a 150% decline in battery output on the same load within a very short period of time Where I live we do not get load shedding due to a substation that cannot be switched off. When I bought the set up in March 2022 it could run my fridge and office for about 15 hours. The station tripped earlier this year and it maintained a similar performance since it does not cycle much due to no load shedding. When it tripped again in March this year, I suddenly saw a massive drop to only 1 hour. Thinking it was a fluke I left it there. However, with the weekend's areas outage, I again found that it only lasted 1 hour. This cannot be acceptable. My circumstances are different to the average load shedded user so with very few cycles how can I go from approx 15 hrs in Jan to 1 hour in March with no outages in between? I bought the entire kit, housing included at great expense with the clear understanding from PSS that AGM batteries are the best option and are good for a number of years. Clearly not so. When I enquired all I got was a quote for Lithium and a sarcastic comment about why I need a system if I don't get load shedding in the first place!!!