Best way to serve videos for (React Native) mobile app: why is HLS so slow over LTE?

Question

I would like to serve video files in my mobile app (React Native using the react-native-video player). The two things I care about are:

1. Adaptive bitrate so people with slower bandwidth (e.g., LTE) can still watch the video with minimal buffering
2. Fast seek times
3. Low bandwidth consumption

It seems like HLS streaming (specifically an .m3u8 playlist + TS files) fits all the above criteria.

I used ffmpeg with -hls_time 6 (6 second segment lengths, as recommended by Apple) to generate the files. Here's an example of a master playlist:

# sample-30s.m3u8
#EXTM3U
#EXT-X-VERSION:6
#EXT-X-STREAM-INF:BANDWIDTH=1205600,RESOLUTION=1920x1080,CODECS="avc1.640032,mp4a.40.2"
sample-30s_0.m3u8

#EXT-X-STREAM-INF:BANDWIDTH=105600,RESOLUTION=1280x720,CODECS="avc1.64001f,mp4a.40.2"
sample-30s_1.m3u8

#EXT-X-STREAM-INF:BANDWIDTH=52800,RESOLUTION=854x480,CODECS="avc1.64001f,mp4a.40.2"
sample-30s_2.m3u8

#EXT-X-STREAM-INF:BANDWIDTH=52800,RESOLUTION=640x360,CODECS="avc1.64001e,mp4a.40.2"
sample-30s_3.m3u8

I then store them on DigitalOcean Spaces and source them in react-native-video via the CDN endpoint for the master playlist, e.g.:

https://onecoach-public.nyc3.cdn.digitaloceanspaces.com/classes/beginner-to-2.0/1-ball-control/1-why-ball-control/view0_hls/view0.m3u8

This all works fine and dandy on my 50Mb/s down Wifi, but when I switch over to LTE (~5-10Mb/s from DO's NYC1 datacenter) the HLS stream seems to buffer every 6s (on the boundary of the segments). Each segment is ~800KB, so 5Mb/s should be more than enough to download each segment in time.

Am I doing something wrong here?