Jitsi 1-click droplet - how to upgrade Ubuntu?

Logged in to terminal some time ago and I was persistently offered the option to do-release-upgrade, obviously to Ubuntu 22.04LTS. I wasn’t in a rush, so it is only now that I am trying to do the upgrade. And it won’t!

apt update and apt upgrade both fault that some public keys are not available. For example:

=start=quote=
W: An error occurred during the signature verification. The repository is not updated and the previous index files will be used. GPG error: https://repos.insights.digitalocean.com/apt/do-agent main InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 77B79B3FFAF7EF65
W: An error occurred during the signature verification. The repository is not updated and the previous index files will be used. GPG error: https://repos-droplet.digitalocean.com/apt/droplet-agent main InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 35696F43FC7DB4C2
W: Failed to fetch https://repos.insights.digitalocean.com/apt/do-agent/dists/main/InRelease  The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 77B79B3FFAF7EF65
W: Failed to fetch https://repos-droplet.digitalocean.com/apt/droplet-agent/dists/main/InRelease  The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 35696F43FC7DB4C2
W: Some index files failed to download. They have been ignored, or old ones used instead.
=end=quote=

apt dist-upgrade resolved some of the package held back issues but still the no public key remains as shown in the quote above

Oh dear! Although apt upgrade and apt dist-upgrade has done all they can, it clearly isn’t enough because do-release-upgrade then responds:

=start=quote= Checking for a new Ubuntu release Please install all available updates for your release before upgrading. =end=quote=

Any ideas on how to progress would be welcome.

Thanks in advance.