How can I access a datastore from one place into another place?

Hi! You can accomplish this locally, with TeleportService:GetLocalPlayerTeleportData()
or 2, player:GetJoinData() i would do the second. Both return everything in a table that you teleported with. Example:

local data = {game.PlaceId, "Hello world!"}
game.Players.PlayerAdded:Connect(function(player)
game:GetService('TeleportService'):Teleport(gameId, player, data)
end)

in the second option, you would do this:

game.Players.PlayerAdded:Connect(function(player)
local JoinData = player:GetJoinData()
local Data = JoinData.TeleportData
local SourceGameId = JoinData.SourcePlaceId --this gets the game id of the game you were tp'd from.
print(Data[2]) -- would print the second table in "data:, which is "Hello world!")
game:GetService('TeleportService'):Teleport(Data[1], player) -- would redirect back to the game you were teleported from.
end)

you will have to make it match with your game. I hoped this helped!

Edit: Make sure "Third Party Teleports" are enabled in both games, (game settings, security)