My lo was like this once, her sleeping time was 3-5am till 2-3pm next day (she won’t wake up even when you ping pong piang beside her).
What I did was once bedtime, everyone in the room and cannot leave anymore (I realize when we leave the room, she will wake up halfway). When she starts to whine, I’ll just pop the milk bottle straight then ignore her and pretend to sleep. She’ll still attempt to wake us up/play on her own but once she sees no one is bothering her, she usually falls back asleep within the next 30mins.
I drag her up at 7am then let her nap at 12pm - 1:30pm (max 2pm) then she will stay awake all the way till 8/9pm then repeat the top again. Around 6-7pm she will usually struggle wanting to sleep but I’ll just grab her to shower or offer snacks to keep her awake else she will wake up at 12/1am 😖. Slowly when she gets used to the sleep routine then you slowly adjust to your preferred timings. It took my LO 3 weeks to get used to this btw, definitely not an overnight thing. My LO is underweight like yours, hitting all her milestones too just sleeping abit haywired 😅.