It’s actually not round but more like a slightly misshapen circle.
If you managed to walk on a give line of latitude or longitude you would end up at the same place except for problems such as ocean, mountains and other obstacles.
Everyone has always known the earth was roughly “round”. It isn’t hard to figure out since if it were flat the horizon wouldn’t disappear. The issue in 1492 was the size of the earth (Columbus had it wrong and thought the earth was much smaller, the general scientific consensus was correct as to size, and had the Americas not been there a crew sailing west would have died of thirst and starvation before reaching Asia). No ever thought that anyone believed it was flat (except so not very bright public school teachers, who probably moonlight teaching in day schools).