There is no predefined standard keyboard shortcut to go back to a tab control from inside its content.
IN a native app, you can go to next/previous tab with Ctrl+Tab/Ctrl+Shift+Tab or Ctrl+PageDown/Up, or sometimes Ctrl+number.
However, on the web, these shortcuts are of course already taken to control the tabs of the browser itself. You won't be able to intercept them for your own use, and even if you could, it would be a terribly bad idea because you would prevent the user from changing browser tab.
If your modal dialog is so big and so long, maybe you can think about modifying the behavior of the escape key.
On first press, you go back on the tab control, and if you area already on the tab control then it closes the dialog.
The user whould have then to press escape twice to close the dialog completely. This is a change from standard escape behavior, but it's maybe more acceptable than defining a completely new keyboard shortcut that no one will use because it's unknown or they didn't get the information at proper moment or didn't remember about it.
More generally, it's maybe the sign that your UI design is too complicated and that you should simplify it or organize it differently.
What about splitting into different dialogs (e.g. one per tab) ? Making groups that you can expand/collapse ? etc.
A big part of accessibility is also about making things simple. If it's too complicated to do it accessibly, maybe you should simplify.