signature check failed

The only solution is to add it as a new app.  There is no way to recover your developer key and as you see you need the original key to update an existing app.

The thing you want to do now is save off your new key somewhere so it's safe if something like this happens again

what? really? i have access to my account, but my app dies because it is not possible to re-generate the developer key, or sign a new version with a different one?

It's security.  You must have the original developer key to update an app in the store.  Garmin doesn't have access to the private key, only the public version.

This is why you want to have your developer key saved off in a secure place.

It's stupid, not security as it does not add anything to security :D . People with your login could delete existing apps and replace them with others. It only is stupid and very annoying and not well documented at all. It's maybe the biggest flaw on the developers structure so do us a favor and stop promoting it as security :D.

kinda agreeing with you. The developer account should be the core security block. enforcing strong passwords and 2FA. We should be able to generate and revoke developer_keys. even if it took 1 week to allow the replacement of the developer_key of a given app upon request

is totally unclear that the loss of the developer key means no more updates for the app

Please stop posting AI nonsense

Well, they could delete my apps and create new apps (probably with different manifest id) but only with their developer key (unless they also stole mine), so at least existing users won't get a mallicious update. This is pretty much industry standard.