Package: complete x2go repo
version: none

sudo apt update
The following signatures could not be verified because their public key 
is not available: NO_PUBKEY E1F958385BFE2B6E
W: GPG error: http://packages.x2go.org/debian buster InRelease: The 
following signatures could not be verified because their public key is 
not available: NO_PUBKEY E1F958385BFE2B6E
E: The depot "http://packages.x2go.org/debian buster InRelease" is not 
signed.
N: An update from such a repository cannot be done in a secure way, so 
it is disabled by default.
N: See the apt-secure(8) manual page for more details on package vault 
creation and user configuration.

sudo apt-key adv --recv-keys --keyserver keys.gnupg.net E1F958385BFE2B6E
Executing: /tmp/apt-key-gpghome.4WhtJFIi3f/gpg.1.sh --recv-keys 
--keyserver keys.gnupg.net E1F958385BFE2B6E
gpg: Received from key server failed: The waiting time for the 
connection has expired.

x2go-keyring package is not available for debian buster => would solve 
this issue!