A young person travels in time to post-apocalyptic Britain where the wheel is outlawed. I seem to remember there were people, perhaps monks, ruling from underground. They revered a book of mathematics written by the traveler's teacher. I read it in the 60's or early 70's
Could this be "The Future Took Us" by David Severn?
Two English schoolboys are suddenly transported from the mid-20th century to the end of the third millennium. The Sussex terrain in which they find themselves is the same, but much changed, apparently post-nuclear apocalypse, with a much reduced population living as the Anglo Saxons might have. The boys journey to London, where they quickly become involved in a popular uprising against the ruling elite, who cruelly suppress the proletariat, whilst themselves enjoying a technologically advanced and highly mathematical existence. It was the Calculators who arranged for the boys to be whisked from their headmaster's study at a critical moment - although to a degree that was an accident; their intended target was the headmaster.
Ursula K. Le Guin's short story "The Masters" first published in the February 1963 issue of Fantastic Stories of Imagination and later collected in the excellent anthology The Wind's Twelve Quarters fits very well, with the exception of time travel:
In a post-apocalyptic world, the young protagonist exists in a backward society where even mathematics as sophisticated as the concept of zero are dangerous heresies. He finds the company of an order of semi-monastics who secretly perpetuate math and science, while maintaining a facade of parochial ignorance to the majority of their community. Much of the story revolves around the protagonist gaining access to and flourishing in the world of taboo mathematics.