This sript need to run every time the system is rebooted since the /root/.ssh/authorized_keys overwrites every boot.
This can be accomplished with a boot script. Flow its guide UDM / UDMPro Boot Script

Compatibility

Tested on UDM PRO
UDM Pro doesn't support ed25519 SSH Keys

Installation

The script was tested on UDM PRO

curl https://raw.githubusercontent.com/fire1ce/UDMP-Persistence-SSH-Keys/main/install.sh | bash

Add you public RSA keys to:

/mnt/data/ssh/authorized_keys

Uninstall

Delete the 99-ssh-keys.sh file

/mnt/data/on_boot.d/99-ssh-keys.sh

Delete your authorized_keys file

rm -rf /mnt/data/ssh/authorized_keys

Usage

At boot the script with read the /mnt/data/ssh/authorized_keys file and add the content to UDM's /root/.ssh/authorized_keys

Manual run:

/mnt/data/on_boot.d/99-ssh-keys.sh