Since I was happy with the 4G/LTE connectivity "performance" of the Chinese Beta ROM 9.4.18 I decided to use the Xiaomi.eu ROM 9.4.18 which I assumed is based on the respective Chinese Beta ROM. Both have the same baseband version, AT4.0.c2.6-303-0416_1014_8fadd5d. However, obviously there is a difference because almost immedately after starting to use Xiaomi.eu ROM 9.4.18 I experienced the same 4G/LTE connectivity issues!
Default preferred network is "GSM/WCDMA/LTE" / "LTE/UMTS auto (PRL)" for the Xiaomi.eu ROM 9.4.18 (finally, no TD-SCDMA!). Phone calls and data connection did not work when connected with a 3G (H, H+) network after losing 4G/LTE network connection ("fall back"?). When "dropping" from 4G/LTE VoLTE to a 3G connection voices become garbled and the connection is terminated at some point. So, no real "SRVCC" (
Single Radio Voice Call Continuity) here, also with the Chinese (?) modem.img/baseband version that's being used in the Xiaomi.eu ROM 9.4.18.
I've now changed the preferred network to "LTE/CDMA/UMTS auto (PRL)", since there are neighboring WCDMA cells in addition to the available LTE and GSM cells. However, according to wikipedia UMTS is rather WCDMA-ish, so I am not sure whether this will improve my Mi MIX 2S' connectivity "performance".
Voice quality is very good though, which probably can be related to the VoLTE/HD-Voice standard, when I am connected with 4G/LTE networks.