The Location of Man Marked by flames.
TLDR at the end
The 4 Road Poneglyphs are needed to find the route to the last island Raftel where one piece exists. Three of them exist in the new world part of grandline but location of the last one is unknown. It is a bit strange that Eustas Kidd of all people knows about Man Marked by Flames.
Roger surely knew location of all four. He clearly went to Wano and Zou but did not take the entire poneglyph with him probably because Zou stone is significant to its citizens and Wano is already a closed off island and bringing the stone out of Wano makes no sense. Wano is a closed off island so the stone is safe and secure away from World Government.
We know Roger stole markings of one from Big Mom. The Poneglyh itself was not stolen but just the markings becuase Big Mom had possession of this one when Stawhats went to Wholecake.
My speculation is that Gold Roger carried the final Poneglyph with him and before he surrendered to World Government, he entrusted the stone to Man marked by flames. Man marked by flames is not in the new world. If he was in the new world, his location would surely fall under one of the Yonko territories and they would know his location. Surely they are strong enough to defeat him with their crews if not alone
I think the Man Marked by flames is in a remote location in South Blue. Roger spent time in South Blue with Portgas D Rouge after he abandoned his crew. Eustas Kidd is from South Blue which is how he might have heard the rumors. It is also the most unknown blue. We were never told much about this blue at all. The straw hats will make a journey there as Luffy might hear its voice or Gaban might give them a hint that Roger must have left it in South Blue. Going out of the Grandline through an unknown path will also act as a crucial step for Nami to finish map of the world, Sanji visiting all blue and potentially Brook meeting Laboon during the return journey.
TLDR: I think Man marked by flames is hiding in a remote location in South Blue.