ACT terminates the fibre outside (possibly terminating a few services at once), and only spec their backup battery for 1–1½ hour (I have observed about 1½ hours on what should be a fresh battery).
Other ISPs I’ve used in Hyderabad (Hireach, Airtel) terminate the fibre inside, so you can put whatever backup you like—I have a small ₹1,000 thing which kept the Hireach one going for around 4½ hours, and the Airtel one for 1¾ hours. (Still haven’t got a proper inverter, not so convenient when renting.) The connection always worked until my battery ran out.
Other ISPs I’ve used in Hyderabad (Hireach, Airtel) terminate the fibre inside, so you can put whatever backup you like—I have a small ₹1,000 thing which kept the Hireach one going for around 4½ hours, and the Airtel one for 1¾ hours. (Still haven’t got a proper inverter, not so convenient when renting.) The connection always worked until my battery ran out.