Thanks again @wborn, your answer solved my problem.
For people who’re having the same problem in the future;
Edit or create the file startup.sh via:
sudo vi /usr/local/etc/rc.d/startup.sh
Put the next text into the file:
sudo insmod /lib/modules/usbserial.ko
sudo insmod /lib/modules/ftdi_sio.ko
sudo chmod 777 /lib/modules/usbserial.ko
sudo chmod 777 /lib/modules/ftdi_sio.ko
Change rights of startup.sh:
chmod 700 /usr/local/etc/rc.d/startup.sh
The next time your NAS reboots, modules will be starten automatically.