How to upgrade automake to 1.16.5?

[afernandez]

In a Ubuntu 20.04 system, I had to upgrade autoconf to 2.71, which then 
required upgrading automake to 1.16.5. I'm following the instruction on the 
website but getting the following errors:
wget https://ftp.gnu.org/gnu/automake/automake-1.16.5.tar.xz
wget https://ftp.gnu.org/gnu/automake/automake-1.16.5.tar.xz.sig
gpg --verify automake-1.16.5.tar.xz.sig
gpg: directory '/home/ubuntu/.gnupg' created
gpg: keybox '/home/ubuntu/.gnupg/pubring.kbx' created
gpg: assuming signed data in 'automake-1.16.5.tar.xz'
gpg: Signature made Mon Oct 4 03:23:30 2021 UTC
gpg: using RSA key 155D3FC500C834486D1EEA677FD9FCCB000BEEEE
gpg: Can't check signature: No public key
gpg --keyserver keys.gnupg.net --recv-keys 7FD9FCCB000BEEEE
gpg: keyserver receive failed: Server indicated a failure
Thanks.