I'm not sure exactly what you have loaded on your mangOH Red, so some of this is speculation...
There is a kernel driver (
pca954x) available for the I2C switch at address 0x71. If you do
i2cdetect -y -r 0 and the entry for
0x71 shows up as
UU, then you likely have that kernel module bound to that device. If that's the case, then you should also have
/dev/i2c-4. These are virtual I2C buses created by the pca954x. Bus 1 represents downstream port 1 of the i2c switch.
/dev/i2c-0 represents the main i2c bus.
On page 3 of the mangOH Red DV3 schematic we can see the Raspberry Pi header. The I2C lines are labeled
SDA_EXP. If you click on the label, you will be sent to page 4 of the schematic. You can see that
SDA_EXP are coming from
SC3 of the
TCA9546A. The hardware ports numbered 0-3 correspond to virtual buses 1-4, so in this case you should access your sensor using
/dev/i2c-4. The driver for the TCA9546A automatically takes care of writing to the device to enable/disable the correct port when you access one of the virtual i2c buses.