SSH login with private key not working

On a GL_Inet 1800AX router with firmware 4.7.0 I want to login via SSH with private key.

First I installed OpenSSH beside Dropbear as I am more familiar with the first, just another port number 2222.
I tried ssh -vv -p 2222 -i ~/.ssh/id_ed52219 -o ChallengeResponseAuthentication=yes root@IPROUTER, but it keeps asking for a password even after I added the public key id_ed52219.pub to /root/.ssh/authorized_keys' and did ssh-copy-id -p 2222 root@IPROUTER`.

Then I tried as well copying the public key via the LuCI interface for using the dropbear SSH, not OpenSSH: https://IPROUTER:8443/cgi-bin/luci/admin/system/admin/sshkeys, but to no avail.

How can I enable private key logging in ?

Hi I just tried by Lucy on: https://x.x.x.x:8443/cgi-bin/luci/admin/system/admin/sshkeys and I pasted my .pub key, it worked. I use Dropbear

Weird, for me this is not working, it still prompts for a password.
I saw that the file /etc/dropbear/authorized_keys is updated with the public key as result of adding the pubkey in the LuCI page.
This is the LuCI page for Dropbear settings:

Is here anything wrong ?