Confusion with iso verification

[mahesh]

Hello. I hope everyone is well and healthy.

Ubuntu 20.04 here and I need to rebuild machine.

Downloaded ubuntu-20.04.3-desktop-amd64.iso
Downloaded SHA256SUMS (current nautilus doens't acutally download the file so opened and pasted into text file, named SHA256SUMS)
Downloaded SHA256SUMS.gpg

run commands:
gpg --keyid-format long --verify SHA256SUMS.gpg SHA256SUMS
gpg: Signature made Thu 26 Aug 2021 05:52:49 AM EDT
gpg: using RSA key 843938DF228D22F7B3742BC0D94AA3F0EFE21092
gpg: Can't check signature: No public key

The tutorial assumes there are two key ID's present in the faulty output - RSA and DSA. Yet when I run this there is only the above and this does not match anything in the tutorial.

At this point I don't know what to do.

Any help much apprciated.