Since:
it arrived only about 3 days ago,
the battery was around 70% on delivery,
it has not charged at all for three days,
you restarted it,
and you tested it with a known-good charger from the exact same model,
you should not spend more time troubleshooting or opening the device. Because it is essentially new, I would request a replacement or immediate warranty resolution, rather than accepting a repair if Microsoft's return/replacement window still applies.