Sonic Frontiers stage 1-7 is one of many early game stages you can find among the Kronos Islands.
However, unlike other Sonic Frontiers stages which can be easily accessed on foot, stage 1-7 is on an island to the south of the landmass and seemingly inaccessible.
There is a small puzzle involved in reaching them - which not only allows you to reach stage 1-7, but several other items at the same time.
How to reach stage 1-7 in Sonic Frontiers
As you explore and unlock challenges, you gradually begin to unlock more of the map. This will soon include stage 1-7 - which is located on an island to the south of the Kronos Island area.
However, it's seemingly inaccessible - with no rails or platforms leading over to the island.
To reach stage 1-7, you have to find a puzzle switch on the cliffs to the east. It'll have three poles nearby, which you must interact with first, creating a tether between them.
Do so from left to right, or right to left - as long as you do the middle pole second - recreating the pattern that is on the switch.
Now press the puzzle switch - doing so will see it create a rail you can use to ride across to the left-most island.
Once there, you have another of these puzzles. This one is easier - it's just the two poles to connect. Do this, then activate the switch, to create another rail - this time from the previous cliff over to the island we are aiming for.
Ride back and across, and you can now access stage 1-7.
Also here are several Fishing Tokens, a side-quest, and a switch which creates a spring so you can return to the mainland.
Best of luck getting all the keys from the stage!