Try track 6 - the early Piano Sonata in C minor, in one movement (1877):
https://www.prestomusic.com/classical/products/9750224--rontgen-piano-music-vol-6
It's a superb disc!
I'm not sure that Röntgen was a great composer, but he was certainly a supreme craftsman. Is that fair?
There aren't that many "great" composers around anyway, and I'd maintain that Röntgen could also occasionally surprise (in the best way, I mean), so/and yes, fair ;)
Concertzender - don't know if they still archive old programs - had something like 20 hours of broadcasts, a version of the Composer of the Week idea but bigger but with much fewer in-studio recording, in those archives one could find by search many older CD recordings of his music, which made for a very good introduction, imho.