You are using APE v1 tags in mp3. So the tags are at the end of the file (not really common case). But since you are enabling playback cache the playback starts before the file is fully downloaded, so Symfonium have no access to the tags to apply the RG.
I would recommend to switch to ID3V2 tags that would work for this use case.
I also tried using the files directly on my phone and there it works, although the volume is reduced a lot compared to Spotify, YouTube and other apps. Is the target for replay gain 89db and it’s there a way to increase it in the app?
The replay gain values are applied as given in the tags for the RG values. The other even less standard tags are not used.
So if you normalize everything to a lower DB target to avoid any possible clipping you need to use the EQ/DSP from Symfonium to increase the volume, like with the pre amp. If you want to push the volume higher you can even do it and enable the limiter to avoid any possible clipping.
You might want to enable AutoEQ as it might surprise you too.