no public key found for ppa

[brainchild0]

After following the instructions for adding the PPA to my local system, I encounter the following message from the output of apt update:

Err:36 http://ppa.launchpad.net/numix/ppa/ubuntu jammy InRelease
  The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 52B709720F164EEB
Hit:38 http://ppa.launchpad.net/ubuntu-mozilla-daily/ppa/ubuntu jammy InRelease
Reading package lists... Done
W: GPG error: http://ppa.launchpad.net/numix/ppa/ubuntu jammy InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 52B709720F164EEB
E: The repository 'http://ppa.launchpad.net/numix/ppa/ubuntu jammy InRelease' is not signed.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.

The system is Linux Mint 21, which is based on Ubuntu 22.04 (jammy).

[BlueManCZ]

Can you try this command? It should basically download the public key you are missing:

sudo apt-key adv --recv-key --keyserver keyserver.ubuntu.com 52B709720F164EEB