It may depend on the printer itself - what it defaults to based on what app you are using to print. Check the settings on the printer and see if you can make the A4 tray be the default for any printing [whether from the iPad or from your other computer(s)].
Or, if you are trying to print a photo, don't send it to the printer from the Photo app. Maybe put it into a PDF file and then print the PDF. That may "convince" the printer to use the A4 tray versus the photo tray (as it won't know that are printing a photo.
Another or: check with Epson? There are Epson apps in the AppStore. I have an Epson printer and I can use the app to print. It's not a photo printer, specifically, however.
Hopefully, one of these ideas will help. If not, maybe someone else will be along with another solution...
Marilyn