Gibson les paul custom's string action(string height)?

I used to play my les paul custom with all original hardwares in Korea with strings that are 2mm(05 ⁄ 64in) above fretboard which is located in the middle of 11th and 12th frets. As soon as I went back to Chicago, I changed the tuners to hipshot black locking tuners and bridge to tonepros black tune o matic. I asked the guitar tech to set my guitar strings 2mm above the fretboard in the middle of 11th and 12th frets. And he did. It played well in Chicago. When I came back to Seoul, I couldn't hear 20th fret's sound at all. So I took it to a guitar repairing shop to see what's going on. But everyone was telling me that the strings were too low or to change the nut and level the frets. Are there any ways to play my guitar with strings located 2mm above the fretboard?