Rules:
*You can teleport into and out of it at will
*It has a couple of plug sockets and can connect to internet from the region you teleported in from
*You can take objects and people with you
*As already stated, it is (3m)^3 (3m*3m*3m). The walls are plain plaster with a light in the middle of the ceiling. The pocket dimension is topologically toroidal, so if there weren’t walls and a ceiling/floor (which you can actually destroy) you would loop if you went more than 3m in any direction. Gravity, then, is artificial and can be altered to anywhere from 0 to 2g from a dial on the wall.
Edit: additional specifications
*You can only teleport out to where you teleported in from.
*Time proceeds at the same rate inside the pocket dimension
*There is an eject button for those inside to get out if something happens to you
The national grid. Using the same portal to the real world that the Internet connectivity uses.
Power over fiber optic cables?
No, power over electrical cables, internet over mobile data/WiFi, if the place you teleported from has that.
How do the cables enter the toroidal space?
A tiny portal.
Who’s paying for it? Is the Wi-Fi also entering through this portal? What other electromagnetic incursions can we expect?
I guess the power companies? But they won’t notice such a small usage. Yes, that is also where mobile data/WiFi enters the pocket dimension, from the place where you teleported in.
So you’re stealing power, and there’s definitely other EM leakage that you’re conspicuously not mentioning.