I still think that if they were created as online characters in the first place that they still are and the UI is just having issues. You can use a site like lastepochtools.com to load online characters without the game. It reads directly from the API that sees the database. Cut the game engine out of the equation.
If it doesn't show up there, I really don't think they were made as online characters in the first place.
You can also check the offline save file location on your computer. If the saves are there then I also don't think they were created as online characters.
It's just one of those things that would require a very significant amount of code that just isn't in the game. I do have first hand experience in this as I was directly involved with making this in the first place.
You can also contact support and they can probably look up your account to see if it has any characters saved online. support.lastepoch.com