Problem: CS510 series won't pull paper from any available tray automatically. Instead, it says something like "Load Tray 1 Plain A4", even though Tray 2 has paper in it. Or, maybe, vice versa.
Related: You've loaded the all trays / the only tray with paper, but the printer says you need to load it again, saying something like "Load Tray 1 Plain A4" or similar.
Solution: The CS510 doesn't auto-detect the loaded paper size(s) from the trays directly. Instead, you have to go to Settings/Paper Menu in the printer web server to set the paper sizes.
It will then "know" that one or more trays are holding the correct paper size for your print job. It will then stop complaining that you need to load a particular tray with a particular size.
This will also fix cases where you've loaded letter and print to letter, but the printer "thinks" the tray holds A4, or similar mix-ups.
This will obviously not work if a multi-tray printer is loaded with different paper sizes for different jobs. You can't expect, nor likely want, the printer to print letter size when the only available paper is legal size (or vice versa).
This shouldn't be an issue with a printer that's been in use because it presumably has been set up before, and will therefore "know" what paper has been loaded. But a new printer will assume all the trays are European A4 size. Which is just blinding if you are in Europe (or the UK/Switzerland/Norway/etc.) but not so great if you are over the pond.