A likely problem is that you are not setting the git ssh port to 3122 on clone. On Asustor NAS, Gitea runs in a Docker container and maps port 22 to 3122 so as not to conflict to the NAS ssh service. This means you need to tell git to use 3122:
git clone ssh://git@host:3122/user/repo.git
This will connect to the ssh server in the gitea Docker and everything will work. The git user is in the Docker image and is preconfigured to “just work” as long as you connect to the gitea ssh server on port 3122. Note that the “ssh://” is required. Once you have the repo cloned you’ll never have to worry about the alternate port number as the repo config will have it mapped.
You can get better insite into the Docker setup by installing “portainer.io” from the NAS app center. This app will show you all your containers and their mapped ports.