Wrong key when installing graph tool

When I install a new Ubuntu on a device and want to install graph-tool,
following the instructions:
at the moment with bionic version of Ubuntu (18.04), but I always get the
error message:
W: GPG error: http://downloads.skewed.de/apt/bionic bionic Release: The
following signatures couldn't be verified because the public key is not
available: NO_PUBKEY 7A80C8ED4FCCBE09
If I run the:

If you want to verify the packages, you should use the public key
612DEFB798507F25, which can be done with the command:

apt-key adv --keyserver pgp.skewed.de --recv-key 612DEFB798507F25

command, I still get the same error.
If I change the key to the one it is complaining about,

still complaining.

What do I have to do?

attachment.html (1.94 KB)