I have had difficulty getting remote keys using GUIs for some reason, no matter what server I try. I am more familiar with Kleopatra, so I use it, but I have tried Seahorse too. (My version, 3.30.1.1, will not take hkps, at least in the Preferences GUI.)
I think the previous suggestion was to try the CLI. Do something like:
gpg --keyserver hkps://keys.openpgp.org --list-keys Adele
You should see something like:
(snipped)
pub dsa1024 2002-03-06 [SCA]
**** DD878C06E8C2BEDDD4A440D3E573346992AB3FF7****
uid [ unknown] Adele (The friendly OpenPGP email robot) adele-en@gnupp.de
uid [ unknown] Adele (Der freundliche E-Mail-Roboter) adele@gnupp.de
sub elg1024 2002-03-06 [E]
That will tell you if you can at least get to the suggested server ok.
As for Thunderbird, I agree with @lithosphere9 that Mozilla completely screwed it up with the integration and dispensing with Enigmail. I was not happy, and I pretty much do not use the encryption implementation anymore. After version 68, it keeps its own list of keys, etc., it does not integrate with anything else that I can tell, and it is more difficult to use. (If you read through this forum, you’ll see a suggestion that the Master Password for that keystore is not really secured. I confirmed this elsewhere, but it may have been fixed by now.)
EDIT: I was able to use hkps://keys.openpgp.org with Kleopatra (Settings->Configure Kleopatra->OpenPGP Keyserver) and get an OpenVPN key back using F554A3687412CFFEBDEFE0A312F5F7B42F2B01E7. Maybe try that instead of Seahorse, especially if you are using amber like I am.