I'm not sure of a single specific PoD, but there are three main things that doomed the rebels.
1) Organisation: By nature the rebellion was sporadic and haphazard, which is to be expected, but for success, the rebels needed to have a more coordinated structure with more high level officers defecting, which also could facilitate the princely states to switch sides. Hyderabad and the Maratha princes of Gwalior and Baroda are a must to get on the rebel side.
2) Expansion: the rebels were limited to the Gangetic plain due to various factors, Punjab remained Pro-British due to Sikhs historical enemity with the Mughals, the Bengal was very quickly pacified, and the south lacked the sepoy base for the rebellion to spread. The rebellion needs to be larger, both to deny the EIC it's replacement sepoys and to bolster the rebels ranks (perhaps Duleep Singh isn't sent to London until the rebellion and escapes to proclaim a Sikh revolt)
International Support: Other great powers need to back the rebels to spite the British, perhaps France under Napoleon III and Russian desires to win the great game cause them to back the rebels, possibly with the Ottomans pro-rebel as well