Looks like you don't have any partitions on your sd card. That's why fdisk -l is coming up empty. You could also run the following to see partition info.
root@Pineapple:~# block info
You can make partitions with the fdisk command from your ssh console
root@Pineapple:~# fdisk /dev/sda
That should bring you to an interactive fdisk console where you can add some partitions. Probably one for your ext4 and a smaller one for a swap.
Then you can use the following to set up your new partitions
mkfs.ext4 /dev/sda1
mkswap /dev/sda2
After all that I would run that block command again to see your progress
root@Pineapple:~# block info
/dev/mtdblock2: UUID="aaa5c26b-1a2d7b95-b821284f-fb82c9e4" VERSION="4.0" MOUNT="/rom" TYPE="squashfs"
/dev/mtdblock3: MOUNT="/overlay" TYPE="jffs2"
/dev/sda1: UUID="080f6ca2-3e2d-4427-88c4-775b1a51da16" VERSION="1.0" MOUNT="/sd" TYPE="ext4"
/dev/sda2: VERSION="1" TYPE="swap"
Then maybe another to see what the config should sort of look like.
root@Pineapple:~# block detect
config 'global'
option anon_swap '0'
option anon_mount '0'
option auto_swap '1'
option auto_mount '1'
option delay_root '5'
option check_fs '0'
config 'mount'
option target '/sd'
option uuid '080f6ca2-3e2d-4427-88c4-775b1a51da16'
option enabled '0'
config 'swap'
option device '/dev/sda2'
option enabled '0'
That should at least point you in the right direction. Hopefully this helps someone. This is by no means all inclusive... and I definitely didn't spend all day working on it. ; )
Check out https://openwrt.org/docs/guide-user/storage/fstab for more info.
