Using public key authentication with dropbear ssh client is a little different from regular openssh clients. Dropbear doesn't lookup users .ssh directory for user's private key. You should expicitly say which private key will be used for session with the -i parameter.
ssh -i ~/.ssh/id_rsa admin@xxxx.asuscomm.com
https://www.snbforums.com/threads/dropbear-ssh-without-remote-password.21070/
ssh -i ~/.ssh/id_rsa admin@xxxx.asuscomm.com
https://www.snbforums.com/threads/dropbear-ssh-without-remote-password.21070/