Redhat – How to remove this warning “This system is not registered to Red Hat Subscription Management. You can use subscription-manager to register.”

Try to re-configure em completely

emctl stop dbconsole
emca -repos drop
emca -repos create
emca -deconfig dbcontrol db
emca -config dbcontrol db

The best regards

You can determine the requirements/dependencies of a particular package with yum deplist packagename.

That doesn't fully solve your problem, though. Do you have access to a Red Hat DVD? Can you bring that with you?

Another option is to try yumdownloader (part of the yum-utils) package. Install yum-utils on a server with internet access and run yumdownloader packagename to download the rpms and depedency packages to your current working directory. You could then take the resulting files to the server that does not have internet access.

If your organization does not provide internet access, it is still possible to use a local repository on your network...