Debian 10.3

installation from https://bird.network.cz/?download&tdir=debian/

apt-key add - ..... OK

~# apt update

Err:2 http://bird.network.cz/debian buster InRelease
  The following signatures couldn't be verified because the public key is not available: NO_PUBKEY B188E2B695BD4743
Reading package lists... Done
W: GPG error: http://bird.network.cz/debian buster InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY B188E2B695BD4743
E: The repository 'http://bird.network.cz/debian buster InRelease' is not signed.