I have the Epson SX415 all in one attached to my DS212j. Printing was no problem from the start, but it took me ages to work out how to use the scanner function, this is how i did it.
1. Make sure desktop drivers are installed.
2. Open the synology assistant desktop software.
3. On the printer device tab, click attach scan/FAX.
4. Windows should now automatically install the drivers.
5. Open Epson scan settings desktop software, ensure the scanner is set to local, not network, then close scanner settings.
6. Open Epson scan software, and you should be able to scan!
You will need to attach scan/FAX each time the PC or NAS is reset or turned off.
Note: this does not install twain drivers for applications such as photoshop. You can only scan using the Epson scan utility.