The hotspot works every time, it's the only thing that is consistent. Hold the button down until it comes on then tap the screen icon "UNLOCK". Matter of fact it's the only thing I use because the surface cell is so flaky.
Something however is going on with the supply side of the Surface because the loaners are often all out and replacement requests are going unfilled??
By the way if your screen is discolored around the edges that is an un-serviceable EFB.