I managed to mount the disk by following these steps:
Connect via ssh as root:
ssh root@192.168.1.1
Find where my disk was by listing the block devices (in my case sda):
lsblk
Format the disk with BTRFS (again in my case sda):
mkfs.btrfs /dev/sda
And finally mount the disk (again in my case sda):
mount /dev/sda /srv
Then I made the final steps in LuCi to access the disk from my computer
Finally, make sure to change the permissions so that you can read and write on the disk:
chmod 777 /mnt/nas