OpenWrt's SSH server is Dropbear. It can accept normal RSA keys. But the authorized_keys location is not the same as the openssh "~/.ssh/authorized_keys".
The location for the authorized_keys is
What you need to do is to put your public key to that file. If you use the default key location
~/.ssh/id_ras.pub, you can do it by this command:
ssh root@YOUR_OPENWRT_ROUTER \
cp /etc/dropbear/authorized_keys /etc/dropbear/authorized_keys.bak
cat ~/.ssh/id_rsa.pub \
| ssh root@YOUR_OPENWRT_ROUTER "cat >> /etc/dropbear/authorized_keys"