The instructions do indeed say to use the rear tray, and using Canon Easy-PhotoPrint Editor.
However, I've never printed photos with Easy-PhotoPrint Editor. I've always used the Photos app. I seldom print photos but I've successfully done it on my Canon TS8360.
Easy-PhotoPrint Editor has options when printing, which i went through to report here:
Under the print options for pages All/from-to you will see a dropdown. The first and therefore the visible option is Page Attributes. Click on this and select Media and Quality
Then - Feed From: Rear Tray
Media Type: Glossy Photo Paper
Using the Photos app I've used the standard 15x10cm (6x4 inch) photo paper in the rear tray. The sliders at the side will secure the paper in the centre. When printing I selected the 6x4 option. The shiny side of the photo paper goes face up in the rear tray and oriented as portrait. When the rear tray is open it seems to override the front paper tray.
At first I failed because I forgot to pull out the printing tray (which receives the printed paper) and got an error message that the blank paper tray needed attention which sent me off in the wrong direction to fix the problem. Maybe this was your experience?