I don't understand about mobile data and 2 sim cards

[marymarymary]

I have learned that because I am now using Android 7 I cannot disable one of my SIM cards but I would like clarification please as to which mobile data will work when I have both enabled . I live in the UK. When I go to Russia in a couple of weeks, how can I ensure my Russian mobile data works and not my UK one (which of course would be very expensive)? I don’t want to take the UK SIM card out because I want to receive texts and possible phone calls.
Thanks for helping a non-techie!

[TobiasF]

There are four different settings for that:

1. “Settings” → “Wireless & networks” → “SIM cards” → “Mobile data”: Here do you actually set which SIM card your phone should use for data transfer when not in a Wi-Fi. The other SIM card will not be used.
2. “Settings” → “Wireless & networks” → “Data usage” → (for each SIM card) “Mobile data”: Here you can enable and disable mobile data usage for each SIM card. That is e. g. important when you have no data plan and data is very expensive. And it will prevent that data is used when setting (1) changes by itself.
3. “Settings” → “Wireless & networks” → “More” → “Mobile networks” → (SIM card) → “Preferred network type”: That has not really to do with enabling/disabling data usage. Here, you can set which SIM can use 3G/4G; this should normally be set to the data SIM card. Set both SIM cards to 2G only before enabling 3G/4G for the other SIM card.
4. “Settings” → “Wireless & networks” → “More” → “Mobile networks” → (SIM card) → “Data roaming”: Here you can set if the SIM card should be used for data roaming (e. g. in foreign countries, on ships and planes). You can only completely enable/disable data roaming and not use a “whitelist” with e. g. EU networks.

[AlbertJP]

I think setting ‘1’ determines which SIM you can control in the drawer/notification bar. You want to disable UK data in ‘2’, set the Russian SIM to 3G/4G at ‘3’, and finally set ‘1’ to the Russian SIM so you can use the drawer to turn off data whenever you want to save battery/not receive WhatsApp.

[marymarymary]

Thankyou to AlbertJP and thankyou to TobiasF

[TobiasF]

@AlbertJP

I think setting ‘1’ determines which SIM you can control in the drawer/notification bar.

As I said, it also determines which SIM is used for mobile data. The other SIM will not be used.

That symbol in the pull-down menu shows the SIM which is used for data and is only there to turn on/off mobile data and view data usage.

@AlbertJP

You want to disable UK data in ‘2’

I think the setting (4) will be more useful as it will automatically disable mobile data when not in the home country of the SIM card.

@AlbertJP

set the Russian SIM to 3G/4G at ‘3’, and finally set ‘1’ to the Russian SIM

That is the procedure when arriving in Russia.

[AlbertJP]

@TobiasF

As I said, it also determines which SIM is used for mobile data. The other SIM will not be used.

Thanks for clarifying, last time I left Europe in 2017 it wasn’t entirely intuitive to me that data on the other SIM actually got disabled when you just changed the ‘preferred’ SIM to another, because you are still able to enable/disable data separately for both SIMs.

At the time, the icons in the top right didn’t display the data connection status when roaming because the 3G/4G got replaced by an R. With Android 7, this is no longer a problem.