Roku Streaming Stick Plus, mp4 with embedded Subtitles not working

Embedded SRT subtitles  is the only way I can use subtitles over my DLNA/UPnP media server

And it's the only way LG webOS TVs can handle subtitles over a network from DLNA servers. I have 6 of these of various ages, all work perfectly 

MKV is not fully supported on LG webOS TVs  

MP4 embedded subs also work on Android TV devices. I have 4 Google TV streamers, and they even work on an old Hitachi Vestel smart TV that I have 

I use option 1 Soft Subtitles (Embedded as a track) . This can easily be achieved using HandBrake or, much faster, using Shutter Encoder rewrap  

Yes, MP4 files can support embedded SRT subtitle files, but there are two main ways this can happen, and they behave differently:

1. Soft Subtitles (Embedded as a track):
   • This is where the SRT subtitle data is included as a separate track within the MP4 container.
   • Viewers can turn these subtitles on or off during playback in a compatible media player (like VLC).
   • The video itself is not re-encoded, so there's no quality loss.
   • To achieve this, you typically use tools like FFmpeg or MKVToolNix (even though it's MKV-centric, it can sometimes work for MP4s or you might convert from MKV to MP4).
   • The specific format for these embedded text subtitles in MP4 is usually "mov_text" (also known as MPEG-4 Part 17 Timed Text or tx3g).
2. Hard Subtitles (Burned-in):
   • This means the subtitles are permanently encoded into the video frames themselves.
   • You cannot turn them off during playback.
   • This process requires re-encoding the video, which can take time and may result in a slight loss of video quality, depending on the settings used.
   • This is often done with video editing software (like Adobe Premiere Pro, DaVinci Resolve, Camtasia) or video converters (like HandBrake, VideoProc).
   • This method is useful if you want to ensure the subtitles are always visible, especially for platforms that don't support soft subtitles or external subtitle files (e.g., some social media platforms).

In summary:

• MP4 supports SRT subtitles.
• You can embed them as a "soft" track (toggle on/off) or "hardcode" them (always visible).
• The method you choose depends on your needs for flexibility and quality.

In short have dozens of files that play on everything except this Roku stick, that leads me to believe there is some deficiency in the software on the Roku stick