Stock S6 on UK 3 network how to set to 2G only network?

Hi all,

I'm on UK's 3 (Three) network and just took an upgrade to S6. Since I'm going to get new Nexus5 I gave the phone to my wife since 3 gives new S6 unlocked my wife is with Vodafone.
The problem is Vodafone sucks in my area and in certain places in my home there is no or very poor network connection. With her previous S5 we selected to use 2G only and she gets full signal.
The problem with S6 you can not chose 2G mode there is no such option as on S5 was. I only get these:

• LTE/WCDMA/GSM
  (auto connect)

• WCDMA/GSM
  (auto connect)

• WCDMA only

There is no GSM only option as it was on S5. And since WCDMA/GSM is auto connect it prioritizes 3G which is very poor.

About the phone:
All stock from 3 (Three UK) used with Vodafone SIM.
Model: SM-G920F
Android: 5.1.1
Baseband: G920FXXU2BOGE
Kernel: 3.10.61-5012324
Build: LMY47X.G920FXXU2BOG8

Is there any way to force 2G only mode without rooting the phone or changing the ROM?
I tried Telinfo app it doesn't launch giving the error message: Your device does not have the required native Google API. (S) And this app works no problem on my Nexus5 with 5.1.1
When I try to input *#*#4636#*#* in dallier nothing happens.

Please Help!

Thank You