Which "antique land" and whose statue does Shelley refer to in "Ozymandias"?

It isn't specified exactly which "antique land" is being referred to in the poem. In this particular context "antique" simply means "ancient," as in ancient Egypt, Greece, or Rome. It's possible that the traveler mentioned in the opening line has just come back from Egypt, which is where he saw the ruins of Ozymandias's statue decaying in the sand.
Ozymandias is an alternative name for Ramses II, an Egyptian pharaoh. He believed himself to be a great ruler, so great in fact that he was certain his name and all his earthly achievements would live on forever, inspiring awe in each succeeding generation. But he was profoundly mistaken. He is not just long-dead but also long-forgotten. The last remnants of his earthly rule lie crumbling in the desert. Ozymandias was so arrogant, so blind to reality, that he was unable to see that everything in this world is transient and will one day fade away.
https://www.poetryfoundation.org/poems/46565/ozymandias