Well it works both ways (in default and LTE/UMTS auto), you can see for yourself even in good 4G network when you call someone it switches to 3G for calling (unless you are using VOLTE). I still think the setting should be LTE/UMTS auto PRL. I see no use of the TD-SCDMA since it is it not in use in Europe, but if there is no harm then who cares, the question is is there any battery penalty for looking for it and are there any negatives in not using PRL. Can someone explain so that we learn something, perhaps @Igor EisbergFollowing up my own message:
I've searched more deeply for the many references to "WCDMA" and have found a very relevant post from somebody who knows what they are saying. I quote without comment, beyond saying that I don't have the required knowledge for anything deep.:
https://xiaomi.eu/community/threads/20-5-28.56038/post-546963 (post 366)
issue ... for MI 9:
1. preferred network type is LTE/TD-SCDMA/UMTS
it's suitable only for China!
https://xiaomi.eu/community/threads/20-5-28.56038/post-546971 (post 370)
Igor Eisberg, Developer, Staff member
You're completely clueless to how this works. You have nothing to do in the hidden Test menu.
"Prefer 4G" or "Prefer LTE" is an auto-switching mode between TD-SCDMA/GSM/WCDMA/LTE.
So my post may have been due to misunderstanding what happens. It does clearly appear that the Chinese network is selected, but that may be deceptive. My concrete suggestion is: if you think you have this problem, do some controlled tests with TD-SCDMA and WCDMA to see if there is any measurable difference in performance where there is no good LTE/4G signal. I'm always in a good LTE area, so can't test.
If my post was misleading, I apologise. And, of course, thanks to the developers for their excellent work on developing this ROM.