Ok, I have found the problem. I didn’t understand well how the GPIO works.
The following configure script resolved my issue:
# Anemometer
config-pin P9.24 uart # UART1 TX (not connected)
config-pin P9.26 uart # UART1 RX
config-pin P8.17 gpio # Enable the sensor
echo "out" > /sys/class/gpio/gpio27/direction
echo "1" > /sys/class/gpio/gpio27/value
stty -F /dev/ttyS1 sane # Configure the serial link
# Modem
config-pin P8.37 uart # UART5 TX
config-pin P8.38 uart # UART5 RX
config-pin P8.15 gpio # Enable the modem
echo "out" > /sys/class/gpio/gpio47/direction
echo "1" > /sys/class/gpio/gpio47/value