GPG key debian repo

If I try to add our repo apt shows:

Get:1 https://dl.jami.net/ring-nightly/debian_9 ring InRelease [2169 B]
Get:2 http://security-cdn.debian.org/debian-security buster/updates InRelease [39.1 kB]
Err:1 https://dl.jami.net/ring-nightly/debian_9 ring InRelease
  The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 64CD5FA175348F84

That key is a selfsig available on public gpg servers, example:

http://keyserver.ubuntu.com/pks/lookup?search=0xA295D773307D25A33AE72F2F64CD5FA175348F84&fingerprint=on&op=index

Please investigate and take the required actions to ensure that apt can verify our GPG key.