Why can't Samsung Galaxy S3 SHV-E3210L catch 3G (WCDMA) network?

Question

Android

Hello! About 4 months ago I bought a Samsung Galaxy S3 SHV-E210L smartphone (Korean version of Samsung Galaxy S3 I9300). OS version: Android 4.4.4; Operator: Megafon
For some reason, it does not catch the 3G network anywhere, although the smartphone LG Spirit (Android 5.0.1) copes with this task.
I will describe the procedure:
1. I check if there is a 3G network on LG Spirit.
2. If there is a network, then I remove the SIM from LG Spirit and insert it into the Samsing Galaxy S3
3. unfortunately, it is impossible to switch the network mode through the usual menu, by typing the command * # 9090 # in the dialer I get to the service menu and selecting Network Mode Set > WCDMA Enable > Enable in it, I switch the network mode to WCDMA in it, and it does not work at all catches! Those. instead of the network level, it shows a prohibition sign, and Netmonitor (I call it with the command * # 0011 # in the dialer) is generally silent. At the same time, GSM is caught perfectly. What could be the reason and how to fix the situation?