How to connect Epson do-2100 printer to my iPhone?

Hi,

Please check the printer model. Do you mean an Epson XP-2100?

If so, it does not appear to be an AirPrint-enabled printer (About AirPrint - Apple Support).

This means that you will need an app from the printer manufacturer:

https://apps.apple.com/us/app/epson-iprint/id326876192

(an XP-2100 series is mentioned under supported models in https://support.epson.net/appinfo/iprint/en/).

The printer and the iPhone need to be on the same WiFi network. Start here: https://support.apple.com/en-us/HT201387