Your isolated people need to be in an area that meets most of the following guidelines:
1) It's very hard to get there. High altitude, lots of natural barriers, or some such. If it's an island, It needs to be way off of the shipping lanes and preferably in a place where there are contrary currents that will take others no where.
2) It needs to be a bit beyond just self sufficient. If you live in the perfect place, why go anywhere else? Especially if leaving means your survival is unlikely. You are going to fall off of that cliff, or you will end up in the doldrums off the coast and run out of water. More than adequate resources will suppress, to an extent, the curiosity of the locals. The community needs to be large enough to keep going with a small amount of genetic diversity. If curiosity is the only reason to leave, not many will want to go. The very curious get selected out, because if they leave, they die. Or they never get back, which is the same thing. Think about Bonobos and Chimps. Chimps are in resource poor areas so they roam about and are aggressive. Bonobos are in a resource rich environment, so they just hang out. Otherwise, the two species are extremely similar.
3) They have nothing of value to the Modern Peoples. At least they have nothing that is worth braving whatever barrier surrounds them. Lets face it, most of the exploring in history was a search for resources. It might have been arable land, food, metals, coal, spices, tea.... you get the picture. If there is no real reason to go to that empty area of ocean that has claimed other ships before, they won't go.
That gives you your basic setup. So long as these conditions remain fairly static, The unknown people are going to remain unknown until Outside technology progresses to the point that it can both overcome the barriers AND for it to be cheap enough to attempt the barriers just to satisfy curiosity. Now we can see evidence of super remote amazonian tribes because we have satellites that can take high resolution pictures without it being too expensive in the big scheme of things. I can use google earth to cover a huge amount of the planet without even putting on pants! Now if I put on pants and decide to go there, I can, but it will still be very hard to get there to be face to face with the remote tribe.
This should be sufficient to keep the isolated community isolated until at the very least, powered boats. Maybe even powered flight.