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.