Hi my friend, I would recommend you installing BB ver.29 by using the TWRP recovery. It should sove he 2g dropping problem on Redmi 3S regardless of the rom which you are using. You can download the file "modem_v29.zip" from here:
http://4pda.ru/forum/index.php?showtopic=755032&st=5360#entry52569518
but you must be logged in order to download it.
Apart of that, I did a fastboot install of the latest Global Stable rom downlowded from here:
http://en.miui.com/a-234.html
This rom has a BB ver.29 inside, but in my opinion you do not need to go through installing a new rom - just flash the BB ver.29 on your EU weekly rom, and it should solve the problem! Good luck!