MH370 went off the radar on Penang Island at a range of 200 nm. If that was due to the Earth's curvature, the plane would have had to be at an altitude of around 12,000 feet. That conflicts with the reported altitudes, so I suspect contact was lost for other reasons than the horizon blocking it.
I measure the distance from the antenna to "a point 10 miles past (west of) MEKAR" as 243nm, using Google Earth.
That's where the earth's curvature should limit a line of sight radar for a plane flying at altitude unless there were some special atmospheric conditions.