Change the Dropbear port to an unused/free one on your box and restart Dropbear

uci set dropbear.@dropbear[0].Port=2222
uci commit dropbear
/etc/init.d/dropbear restart

Make sure you have set a root password before rebooting

passwd

Reconnect to your router via SSH on the configured port above

Install the openssh-server

opkg update
opkg install openssh-server

Enable and start OpenSSH server. OpenSSH will listen now on port 22

/etc/init.d/sshd enable
/etc/init.d/sshd start

Reconnect to your router via SSH on port 22

Now you can disable Dropbear

/etc/init.d/dropbear disable
/etc/init.d/dropbear stop

Install the openssh-sftp-server package to install support for the SFTP protocol which SSHFS uses

opkg update
opkg install openssh-sftp-server

Don't forget to configure the openssh server in /etc/ssh/sshd_config