Samsung Galaxy S7 Duos (SM-G930FD) dual sim 4g + 3g mode

Galaxy S7 and S7 Edge have their dual sim models, so called S7 (Edge) Duos, selling in a few countries. If you got one of them with the model number SM-G930FD / SM-G935FD, your S7 is capable to connect to two separate networks at the same time. There are two card slots in the sim tray where two different nano sized sim cards can be placed.

However, for some S7 Duos models (especially selling in India or Middle East), 4g+3g dual mode is not working and forced to change to 4g+2g mode only. While the main sim card can connect to 4g or 3g network with data enabled, the sub sim card can connect to 2g only and not allowed to switch to 3g.

This is because Samsung banned the 2nd sim to connect to 3g network, as 3g was regarded as a data network which would conflict with the main sim's 4g (or 3g) data network connection. However, in these days many countries use 3g network as their main connection carrying not only data but also voice and SMS text messages. For those countries and carriers where 2g network (GSM, GPRS or EDGE) is not available, this can be troublesome as the 2nd sim card can connect to 3g network, which makes the dual sim functionality useless.

If your S7 Duos is not able to turn the 2nd sim to 3g mode and only working in 4g+2g mode, it can be fixed by updating the firmware to different country. As far as I tested, the recent G930FD firmware for Thailand works perfectly without any issue on 4g+3g dual mode. You might want to update your firmware to the latest version and test 4g+3g mode again before changing the firmware, because the issue is sometimes fixed on the latest version.

Be aware that this method needs factory reset and changing CSC code (country and carrier code) of the firmware. If your current CSC code is in multi-CSC and not a default CSC code in the multi-CSC, it will be difficult to return the CSC code back to your original one after changing CSC.

Changing S7's firmware to one of Samsung's official one using ODIN is simple and not tripping Knox. If you want to change the firmware back to the original, it is always possible to return like as it was, except for the case that your original CSC is in multi-CSC.

You can use ODIN to wipe your firmware to another version. Follow the simple steps.
1) Download the latest ODIN program and Samsung USB driver package, and install them on your Windows computer.
2) Download the latest Thailand firmware of S7 Duos (SM-G930FD / G935FD) from SamMobile.com or SamFirm application.
3) Back up all the data in your mobile
4) Turn off the mobile, and turn on again by pushing VolDown + Home + Power key simultaneously.
5) Phone is booted in Downloading mode, press VolUp key to enter the downloading ready mode.
6) Open ODIN, put the downloaded firmware into the ODIN (BL+AP+CP+CSC). Make sure the CSC selection is the CSC file without HOME_ prefix. FYI, HOME_CSC is used only for updating from the same firmware variant which will not need delete all the data.
7) Connect S7 to the computer with USB cable. Connection is detected in ODIN.
8) Start download. Do not change any options in ODIN (only Auto Reboot, F.Reset Time checked, all others unchecked).
9) Once the firmware is downloaded, the phone will be reset and takes a while to boot up.
10) (IMPOTANT!!) An extra factory reset is necessary to make sure your CSC is changed to THL (Thailand version) before using your mobile. Once the phone is booted up and showing the welcome message, just turn it off again, and turn on by pressing VolUP + Home + Power key for a while booting in Recovery Mode. On the Recovery menu, select 'wipe data/factory reset' and 'wipe cache partition', then reboot again.
11) Now, your S7 is entirely in Thailand firmware, which means 4g+3g dual mode is working perfectly.

More details can be found on internet. There are so many guides online explaining how to update S7 firmware using ODIN. Search for S7 ODIN .

Note that 4g+3g dual mode is working only on Thailand firmware with THL CSC, even after 7.0 update. If your CSC is not THL, 4g+3g is not working correctly no matter what your firmware is. This is confirmed as of February 2017, and I wish Samsung will notice and patch it across all regions soon.