How can I update the OS of a Supported System (Debian + Supervised)

Hope I am not missing something obvious here, but I recently had to redo my entire system since it was not longer supported and couldn’t get any updates.
In trying to remain compliant with ADR-0014 I am trying my best not to install any other software than the base OD (Debian 10 on a NUC) so I never installed OpenSSH. I am trying to maintain the system with only what is provided in the Add-On store.
I tried installing SSH & Web Terminal but it’s not letting me update or upgrade as apt-get doesn’t seem to be a recognizable command.
How do you guys keep your base OS update without having to physically be at the machine’s terminal? Would a script be a good idea?

SSH. I run all my system „headless“ and SSH is installed by default.

No, but install package „unattended-upgrades“.