I am afraid that the provided information was not enough to completely understand your setup and the problem. I built up a test demo based on your explanations - it uses your master page and a content page with scrolling content but when I press tab nothing happens. That is why I believe that you should provide some more information about your setup.
What I can suggest is that you open the attached test demo, modify it in order to reproduce the issue and after you succeed, post the full source code of the two pages along with very detailed reproduction instructions and explanations. Once I am able to observe the problem locally I will do my best to help.
the Telerik team