Getting NO_PUBKEY error in Debian Buster

[shyney7]

Followed the install instructions but Im getting the following output from sudo apt update:

Err:10 http://deb.gierens.de stable InRelease
  The following signatures couldn't be verified because the public key is not available: NO_PUBKEY EC29E2090CE3FD43
Reading package lists... Done
W: GPG error: http://deb.gierens.de stable InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY EC29E2090CE3FD43
E: The repository 'http://deb.gierens.de stable 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.

I also tried to specify the architecture with:
deb [arch=x86_64 signed-by=/etc/apt/keyrings/gierens.gpg] http://deb.gierens.de stable main
Inside the /etc/apt/sources.list.d/gierens.list
But got the same result.

[gierens]

Can you show me the output of the following command: cat /etc/apt/keyrings/gierens.gpg | gpg --armor

[shyney7]

@gierens
Sure:

pub   rsa4096 2023-08-17 [SCEA]
      1548BC8A4B4D2688F9B0DAF7EC29E2090CE3FD43
uid           Sandro-Alessio Gierens (aptly key) <sandro@gierens.de>

[shyney7]

@gierens No solution? :/

OS: Siemens Industrial OS 2.1.1 (buster) x86_64
       #######          Host: SIMATIC IPC627E 6AG4131-3GJ32-8AA3
     ###########        Kernel: 4.19.135-cip31-rt13+ind1
    #############       Uptime: 116 days, 2 hours, 10 mins
   ###############      Packages: 1564 (dpkg)
   ################     Shell: fish 3.7.1
  #################     Terminal: /dev/pts/0
#####################   CPU: Intel i7-8700 (12) @ 3.200GHz
#####################   Memory: 3105MiB / 32015MiB

[gierens]

@shyney7 Sorry, I got a little distracted by other things.

Anyway, the key file looks fine, but your neofetch output might actually include an important hint. Your system appears to be on a distro based on Debian 10 buster. I think they still used the old way of storing keys in /etc/apt/trusted.gpg.d/ instead of /etc/apt/keyrings/ back then.

Could you try to either copy the file there cp /etc/apt/keyrings/gierens.gpg /etc/apt/trusted.gpg.d/gierens.gpg or maybe it actually has to be armored there cat /etc/apt/keyrings/gierens.gpg | gpg --armor > /etc/trusted.gpg.d/gierens.gpg. If one of these works you can obviously remove the file in the original location.