ADRV9009 - Accessing to "gain_table_conf" attribute always gives "Permission denied (13)" error

We are using a custom board with 8x ADRV9009 and Petlinux 2019_R2:

root@DPBOARD_2019:~# iio_attr -d
IIO context has 15 devices:
        iio:device0, ams: found 1 device attributes
        iio:device1, hmc7044: found 7 device attributes
        iio:device10, adrv9009-phy-D1-0607: found 17 device attributes
        iio:device11, adrv9009-phy-B0-0203: found 17 device attributes
        iio:device12, axi-adrv9009-rx-hpc: found 0 device attributes
        iio:device13, axi-adrv9009-tx-hpc: found 0 device attributes
        iio:device2, hmc7043l: found 5 device attributes
        iio:device3, hmc7043r: found 5 device attributes
        iio:device4, adrv9009-phy-H3-1415: found 17 device attributes
        iio:device5, adrv9009-phy-G3-1213: found 17 device attributes
        iio:device6, adrv9009-phy-E2-0809: found 17 device attributes
        iio:device7, adrv9009-phy-F2-1011: found 17 device attributes
        iio:device8, adrv9009-phy-A0-0001: found 17 device attributes
        iio:device9, adrv9009-phy-C1-0405: found 17 device attributes
        iio_sysfs_trigger: found 2 device attributes

Accessing to "gain_table_conf" attribute always gives us "Permission denied (13)" error, using different commands:

iio_attr

root@DPBOARD_2019:~# iio_attr -d adrv9009-phy-A0-0001 gain_table_config
ERROR: Permission denied (13)

iio_info

sysfs

cat /sys/devices/platform/amba/ff050000.spi/spi_master/spi2/spi2.3/iio:device8/gain_table_config
cat: can't open '/sys/devices/platform/amba/ff050000.spi/spi_master/spi2/spi2.3/iio:device8/gain_table_config': Permission denied

On our board we are using Petalinux 2019.R2, so libIIO is v.0.21. We also tried to use a different version of libIIO, v.0.24 (the latest), but we got same errors.

We are stuck on this, so we need some help on this, thank you.