Hello buddy, Thanks for supporting
If you have the DLC, you can start additional puzzles on the island by pressing Esc to open the menu and choose challenge puzzles.
To return to the island:
1. Finish all the puzzles ~
2. Create a new save file to respawn ~
3. Quit puzzles, find a door at the starting point of the puzzle

If you have any questions just hit me n our Discord
https://discord.gg/WwQatnx

Thanks for the answer. Later I will try to follow your instructions.