Did Ambedkar believe in God?
Not in the usual sense, and by the end of his life he had built a religion that did without one. The Buddhism he chose and set out in his last book left aside a creator God, an eternal soul and the literal machinery of heaven and rebirth, and put in their place a demand for moral conduct among human beings, tested by reason. Religion, for him, was not about pleasing a deity; it was a code for how people should treat one another, and he judged every faith by that standard.
This was not indifference to religion — he thought a society could not do without one — but a particular idea of what religion is for. He wanted a faith that would not command belief in what could not be shown, would not sanctify a hierarchy of birth, and would rest morality on other people rather than on the supernatural. Buddhism, stripped to its ethics, met the test where the religions organised around a God, in his judgement, did not.
So the answer is that he was a deeply religious man who did not believe in God as ordinarily meant. He held that the question worth asking of any religion is not whether its God is real but whether its rules make people more equal and more humane — and he spent his last years writing a version of Buddhism designed to pass exactly that test.