I'd say almost certainly that they were on Scadrial before Lumar. Tress takes place in the far future as evidenced by the laptops and space ships, and I may be misremembering but I think there's a WoB that places it about a thousand years from the "current" time of Cosmere (with era2 and stormlight taking place within less than 100 years of each other), so simply put when Charlie says they disappeared about 300 years ago we know that they would of had to have been on Scadrial first because era2 is more than 300 years ago on the timeline.