You can add your public keys by going to:
System > Advanced settings > click on the link for LuCI, log in as root and use the same password you normally use to log into the router.
Once you’re logged-into LuCI, go to System > Administration > SSH-Keys . Then, paste your public key into the text box and click the Add key button.
Before disabling root password access, test that you can successfully SSH into your router.
To disable root SSH access via password, log into LuCI again and go to System > Administration > SSH Access . Then, uncheck the “Allow root logins with password” checkbox. (note: you can still log into the GL.iNet GUI or LuCI using your root password.)
I could add some rambling warning about using RSA-based keys (please just use ed25519), and some more rambling about having issues rsync-ing to dropbear over high-latency connections (if you have issues, install ssh-server) - but I’ll leave those for later if you have more questions.