I see scrollbars on the IFRAME that you use to show the external page - I believe what happens is that the modal RadWindow is correctly displayed in the center of the content page, but because only part of that page is visible, you cannot see RadWindow in full. Try setting KeepInScreenBounds=true for the modal RadWindow and see if it will help.
As for the modal RadWindow not covering your whole page - this behavior is expected. RadWindow exists only in the context of the page from which it is created and its modality overlay cannot "go" outside of the boundaries of the IFRAME. If you want RadWindow to cover the whole main page, you need to put it on that page and not in the content one.
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