On the Eurail / Interrail Rail Planner app, Frankfurt-Niederrad station is treated the same as Frankfurt central station (Frankfurt(Main)Hbf), which is very inconvenient.
Try planner search the “from Frankfurt Airport to Frankfurt Central”.
Frankfurt-Niederrad station is certainly close to central station, but it's a completely different station.
I hope for improvement.
There is no one in the Community who can do anything about this so please enter a request with Customer Support through this form.