git says everything-up-to-date when pushing changes to a remote branch

Depending on your version of Git, it may be trying to push branches with matching names, i.e., master to origin/master and remote_branch to origin/remote_branch. If your origin repository doesn't have a branch named mybranch then it thinks there's nothing to update.

To override this default, you can explicitly tell git which branch to use as the source (mybranch) and which to use as the destination on the remote repository (remote_branch):

git push origin mybranch:remote_branch

There's a config option to tell git to push to remote tracking branches by default:

git config --global push.default tracking

I find this more intuitive and I think it's the behavior you're looking for. Checkout the push.default option in the git config man page. Also checkout the Examples section in the git push man page to see how to override the default behavior.