I followed the procedure for the Chinese 1+3T (flashing EU qnc) but it is not working. But maybe in the US variant works.
OnePlus 3T uses SKY77824-11 and SKY77646-51 as PAM(power amplifier module) for FDD/TDD LTE.
SKY77646-51 for B1, B3, B4, B8, B12, B13, B20, B25, B26, B28, B34, B39
SKY77824-11 for B7, B30, B38, B40, B41
Europe Model (A3003) Bands 1/3/5/7/8/20/38/40
China Model (A3010) Bands: 1/3/5/7/8/38/39/40/41
American Model (A3000) Bands: 1/2/4/5/7/8/12/17/30
There have different board layout, although all variants of 1+3T use the same PAM.
I guess the board layout of A3010 should look similar to attach file.
The RF_OUT_B20 pin is not wired on SKY77646-51.
So there's no way to enable B20 without hardware modification.
It's major difference between A3003 and A3010.
B2 signal through RF_OUT_B25 pin on SKY77646-51, because they have similar frequency 1900mhz.
B17 signal through RF_OUT_B13 pin on SKY77646-51, because they have similar frequency 700mhz.
(Maybe B12 and B17 are only available on A3000)
Ref: Mi5 schematics
https://xdaforums.com/showpost.php?p=71773560&postcount=744