What the Rail Planner app shows is irrelevant. It's known to be out of date and incomplete and should therefore never be used to plan. If your train is missing then you can always add it manually. I see e.g. EC135 partially in the RPA for tomorrow, from Leipzig to Hoyerswerda.
These ECs have the same €4 mandatory reservations for international journeys as the other ECs between Germany and Poland. You can book it on intercity.pl. Reservations are optional for domestic journeys in Germany and in Poland. As always,
Seat61 explains how to book.