I have a Pixma MG6220. No matter what I do it won't print from the rear tray from a source on the web. I go to both the quick setting and main tab and select "rear tray." Still prints from cassette. I took out the paper from the cassette. Won't print unti I load paper in the cassette. I even selected a different paper than plain paper and selected rear tray - but it still printed from the cassette.
I selected "continuous autofeed" for a default, removed paper from the cassette - and it still wouldn't print from paper loaded in the rear tray.
It will print from the rear tray if it's a document originating from my computer (or at least I just tried that and it worked). I did not change from plain paper, I only had to select rear tray.
Yes, I realize "save the document to your computer" will be a suggestion, but the website won't let me. It's a "free printable" that is not downloadable.
The issue may also be related to a setting in your web browser. Check to see if your browser has a Page Setup option. This menu may be controlling the paper feed setting.
If the issue persists, please try using a different web browser to test the unit.