I suspect it's the searching for signal. My office is the same (with an old phone. I'm now work from home) and it would rinse my battery with barely any signal.So, having used a few days now it has suddenly got much better. I'm off charge since 7am and am writing at 4pm and I'm at 80%. Not massive usage - 1 hour screen time with a little Pokémon go in between which seems pretty decent to me. I've disabled 5g, have screen on auto refresh rate and running latest firmware. I do wonder if low signal strength causes disproportionate power drain, the office saw me down to 30% by mid day work virtually no usage at all.