I stream over a T-Mobile unlimited data plan and it works just fine. There’s some startup latency but once the track commences there’s very rarely any interruption.
Sound quality versus file playback from a network HD is quite close. I use a dCS Rossini Player/DAC with clock. The unit seems impervious to tweaks upstream such as using fiber connectors, Ethernet filters, etc. It just works well without any fuss. I don’t have an answer as to why the sound of the OPs system is flat at certain times of the day, my understanding is that streaming services are sending packets of data that either arrive intact or don’t arrive at all, so they’re benign when it comes to issues of sound quality?