-snip-After trying to verify, Kleopatra tells me this:
Not enough information to check signature validity.Signed on Thursday, 19 November 2020 10:57:13 CET by
mycryptomixer@protonmail.com (Key ID: 0x77379A5D).
The validity of the signature cannot be verified.
I was able to verify the message using GPG Keychain on macOS.

Then I tried it at Kleopatra. I tried to import the public key via the Tools menu -> Clipboard -> Certificate Import (I see this method is not in the OP). Then verify the message, and here are the results:
