Welcome to the forum jasokah.
You cannot - as of 1.4.6 - use text frames on master pages in the way that you want.
It is best to think of master pages as "page backgrounds" rather than "page templates". The items on a master page can't change between pages (except for things like page numbers etc).
There are two usual ways of doing what you want:
1. Using automatic text frames.
When creating a document, you can set-up your margins as needed and tell Scribus to create automatic text frames that will be created to take up the whole space inside the margins of each page as pages are created.
One problem with this is that it only really works for brand new documents. Another downside is that every new page will have an automatic text frame whether you want one or not and deleting them can cause problems. Other issues have also been reported. Personally I would not advise using automatic text frames.
2. Using the Scrapbook.
You can set-up the text frame(s) on each page - left and right - then select the frame(s) from each page and copy them to the Scrapbook. Then you can drag the frame(s) - the layout - from the Scrapbook as you need them and link the text from frame to frame.
This is more work than using automatic text frames but you are in much more control over what happens. You can also build up a stock of different page types, e.g. text page left, text page right, chapter heading page left, chapter heading page right, etc.
Unfortunately, there is no quick work-flow for what you want to do but using the Scrapbook will save time in the long run.
P.S. Before you import your text, split it into chapters and import each chapter separately. You will encounter many fewer problems doing that than if you import the whole thing in one go.
P.P.S. IMPORTANT (will I think so anyway). Use styles to format your text. It takes a bit of time to learn but you'll thank yourself for it in the end.