If you add an external tool through Site Info as a maintainer in a site then you normally also want to add that tool to site. It used to be the case that you could do this through the external tools helper, but that's no longer the case.
Now you have to use Manage Tools and add the LTI tool there. This is confusing but also has the limitation that you can only add one copy of the tool. For some LTI tools people need to add multiple copies of the tool which is no longer possible.