DLNA Streamer like C5 never uses logins, for iPhone try substreamer that is using the Subsonic API and of course a user name, it is free and you can cache songs for offline use. For Music only ....
For round about 5$ you can buy play:sub very good for music and many options for Playlists and so on, but no Video support, but still my favorite. AVSub for music is also round about 5$ and for Video Option you need an upgrade with additional 5$ so far. AVsub supports also Chromecast and FireTV as an option, so if Roku supports that, may be it is also an solution.
Thats the only way I see, so you have for iphone a user and for the DLNA only guest ....
Substreamer
https://apps.apple.com/de/app/substreamer/id1012991665 AVsub
https://apps.apple.com/de/app/avsub/id923424694play:sub
https://apps.apple.com/de/app/play-sub- ... d955329386