Looking at the sea charts[1] of the archipelago and following a few undersea cables, I think it might be this cabin[2], which roughly matches with a map on GlobalConnects website[3].

Funnily enough, it's right next to a base of the Swedish military.

[1]: https://geokatalog.sjofartsverket.se/kartvisarefyren/

[2]: https://maps.app.goo.gl/6mYHFhaUp7Jzx1X69

[3]: https://globalconnectcarrier.com/our-network/