TWM 3G fallback


I’m using Taiwan Mobile 4G. Usually all is fine. However I noticed that in poor reception areas, it does not fall back to 3G or 2G.
To check this out, I went into the phone settings and changed the preferred network type from 4G to 3G. It doesn’t have data in this mode!
Any idea? Thought 4G is supposed to gracefully fall back to 3G…
Thank you!