This app was mentioned in 4 comments, with an average of 1.25 upvotes
https://play.google.com/store/apps/details?id=com.ather.networkmode
This app will toggle the same menu. If that slider is not selected then that is the issue. Try and slide it over, the hit "trigger carrier provisioning"
I got an app called Network Mode https://play.google.com/store/apps/details?id=com.ather.networkmode and then about half way down the screen, there is a drop down box. Mine defaults to TD-SCDMA/CMDA/UMTS. You'll want to make note of what your's is, as that's what you want to set it to when you aren't wanting to use 3G only. Now to put it to 3G only, scroll to near the top of that dropdown, and select "WCDMA only". That is 3G only. Then when you are done and want 4G back, set it back to that default setting that you made a note of. Works good. Yeah it's not as nice as a toggle button but it's pretty straight forward.
Wow! I used Mobile Network (https://play.google.com/store/apps/details?id=com.ather.networkmode) to modify the preferred network type. It appears that TD-SCDMA/UMTS puts me in LTE. The following leave me in HSPA+: LTE/TD-SCDMA/WCDMA, LTE/TD-SCDMA/UMTS, TD-SCDMA/CDMA/UMTS, GLOBAL/TD-SCDMA. This is great! Thank you for sharing!