Defected Inverter that HiFi doesn’t want to refund me for!!!

I bought an Ellie’s 720W inverter in early October 2022. A few weeks later it started leaking fluid and the performance dropped drastically. I handed in the inverter for assessment and the supplier advised that they want to replace the batteries. I cannot accept this as this inverter is defected and replacing my batteries only to get the same defect again will be extremely inconvenient to me as I work from home and I need my power to stay on. I do not have trust in the brand and on top of it all HiFi keeps ignoring my calls or they keep saying they’ll call me back to let me know what happens. I WANT MY MONEY BACK!!!