You shouldn't need to restore your repo. Just switch back to dev, get rid of your branch, and then git checkout -b sydb-deprecationEnd_2018-10-01 HEAD~5 (make sure HEAD~5 is what you want—if dev isn't at the proper HEAD, you'll get something else). You can't set up a tracking branch unless there is something to track (i.e. a branch on the remote named sydb-deprecationEnd_2018-10-01). When you git push -u it for the first time, a remote branch will be created.
Just FYI, I wouldn't use git reset that way, because you're running the risk of getting confused about where you're at in your history. A git pull will fix it if the remote's HEAD is at or later than your original HEAD, of course, and you won't be able to push until you're in sync with the remote, but still... I think it's better to use git checkout, either creating a new branch right away, using -b <name> or putting it into a "detached HEAD" state, which sounds terrifying, but basically just means you're working on an anonymous branch, where nothing you do matters unless you turn it into a named branch (with git checkout -b <name>). If you mess it up and want to just throw it away, you switch back to dev, and you're done.
Hope that helps,
Hugh