I haven't tried Twonky in years, so can't speak to their current version. But I use Roku Media Player with my DLNA server with no issues. My guess (and that's all it is) is that Twonky isn't following the DLNA standard completely, which could be why your Roku can't see it. RMP sees my Windows Media Server share with no issue, and if I enable DLNA in Plex that too appears. That's why I'm not certain the problem is with Roku.
Nvidia Shield, Roku Stick (3600), Ultra (4640), Premiere (3920), Insignia 720p Roku TV, Sharp 4K Roku TV, Windows 10 Pro x64 running Serviio and Plex on a wired Gigabit network.