Synology comes with a ffmpeg that cannot handle AAC.
See this:
Even if you can do something with the Synology AAC problem, I’d still recommend you move the Roon Core to Windows.
I never suspected your Ethernet network was faulty. I suspected Roon cannot run on link aggregation. That’s a huge difference.
That is not true. Lumin app certainly supports Tidal AAC. If you bought some AAC files and they cannot be played, please don’t buy AAC anymore. You should only buy FLAC / ALAC / WAV / AIFF / DSF / DFF.