Hello,
The ET-2815 is not listed as supported in About AirPrint - Apple Support.
The printer appears to require Epson Smart Panel, at least for some purposes. Are you using this app for iPad?
https://apps.apple.com/us/app/epson-smart-panel/id1477796092?platform=ipad
(https://support.epson.net/appinfo/smartpanel/guide/en/)
You mentioned that photos can be printed, but not some documents. Is this true for other apps/PDF files/web pages? Please provide additional information.
Normally, the preferred connection method for a Wi-Fi printer is to a Wi-Fi router (infrastructure mode). That is, to the same Wi-Fi network as the iPad. Is this the way your printer is connected? Or are you using some kind of Wi-Fi direct connection iPad to printer?
https://download.epson-europe.com/pub/download/6439/epson643974eu.pdf
Sometimes, a power-cycle procedure may help in case of a device discovery problem:
Power off the Wi-Fi router, the printer, the iPad, and all other computers/devices. Wait two to three minutes. Power on the router. Wait two to three minutes. Power on the printer. Wait a minute or two. Power on the iPad. Finally, power on all other computers/devices.