You can't yet define your own breakpoints in Webflow, you have to work with the 4 given to you.
And breakpoints don't target devices, only screen widths.
Tablets in portrait mode nowadays reach and exceeds the basic 940px breakpoint of a desktop view.
So a tablet landscape will get the desktop view.
I understand you need the tablet to show something different than the desktop (or you have no reason to write you post to begin with) right?
— use scripts to show and hide elements based on their class name, depending on the OS
— use scripts to redirect the user to a specific page when a mobile OS is detected. This is unnoticeable and is quite a cool solution if you can deal with duplicated content and a different URL when you're on mobile. (weird analytics data)
I can't give you those scripts, because I don't code but they're quite simple scripts you can fond or make develop.
I have used the redirection one with great success, on a Webflow site.