For those who want to force WCDMA/3G only can do these step:
1. Open dialler and dial ##3282 and press call and field test should pop up
2. Go to WCDMA and press the 3 dot on right bottom of the screen and choose setting.
3. Change network type to 3G only
so far it works for my lumia 620
source : ... https://forum.lowyat.net/topic/2707522/all
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