iPhone Cellular data only works with one SIM active or on Wi-Fi

Question

Level 1

7 points

iPhone Cellular data only works with one SIM active or on Wi-Fi

Pre the end of 2024, I had a physical SIM and an eSIM. Both from different networks.

The eSIM is linked to the phone number I use, for voice calls, etc.

The physical SIM was set for all mobile data, and if I’m in a dead spot for that network it’s got allow data sharing set or I manually get the phone to use cellular / mobile data via the eSIM.

Over the festive season, I suddenly noticed I couldn’t get onto the internet unless on WiFi.

I’ve been to an approved Apple Store, they ran a bunch of tests and my phone is apparently fine.

I went to the cellphone / mobile carrier shops and got a new eSIM and new SIM.

While doing this it was noticed that if I disable either line, then do a data speed test, I get a good cellular data connection. As soon as I put the turned off line, back on, no cellular / mobile data possible.

What is wrong & so how do I correct this ?

Thankyou

Things I’ve already tried:

reset all the carrier settings for both SIMs
turned the phone on and off
new eSIM and SIM (& waited over 24 hours). If one SIM is turned off, the remaining one works 100% fine.
checked I’m on the latest software.

[Re-Titled by Moderator]

iPhone 14 Pro

Posted on Jan 16, 2025 03:56 PM

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Answer 1

Top-ranking reply

Lawrence Finch

Community+ 2025

Level 10

226,672 points

Jan 17, 2025 06:53 AM in response to Rhauri

Please read the ENTIRE link posted by Servant of Cats. It includes information on how to select which line to use for cellular data. By default it is off, so you must turn it on for one line, as described in this excerpt from the link:

Change your cellular data number

One number at a time can use cellular data. To change which number uses cellular data, follow these steps:

Go to Settings.
Tap either Cellular or Mobile Data.
Tap Cellular Data.
Tap the number you want to use cellular data.

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Answer 2

Jan 16, 2025 11:58 PM in response to Rhauri

Note that only one number at a time can use cellular data.

See: Using Dual SIM with an eSIM - Apple Support

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Answer 3

Rhauri Author

Level 1

7 points

Jan 18, 2025 11:30 PM in response to Lawrence Finch

Yes, I've historically done this. I've selected which number can access cellular data.

BUT since the end of December if I have both lines turned on (it doesn't matter which I line I turn off), regardless of which line I have set for Cellular Data and whether I have "Allow Cellular Data Switching" or not, I can't access the internet (ie no cellular data) unless on WiFi. ie the Cellular Data isn't working.

When I turn one of the lines off (it doesn't matter which) I then get full cellular data / internet access.

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Answer 4

Jan 19, 2025 07:28 AM in response to Rhauri

When you turn one off there is only one line for cellular data, so it uses that one. If both lines are on you must select one for cellular data. If that doesn't work contact both carriers and have them reprovision your service.

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Answer 5

Rhauri Author

Level 1

7 points

Jan 17, 2025 05:25 AM in response to Servant of Cats

Yes, I undertand that.

My challenge at the moment is that if I turn on both lines (SIMs) at the same time, I have no cellular internet access :-(

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Answer 6

iYosuua

Level 1

14 points

Jan 30, 2025 12:14 PM in response to Rhauri

Im facing same issue since last month too. Tried even resetting network configuration. Any update?

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Answer 7

Rhauri Author

Level 1

7 points

Jan 19, 2025 12:55 PM in response to Lawrence Finch

Yes, I fully understand that at any one time, the phone will only get data services / internet access via one of the SIM’s.

My problem is that pre the end of December, I had a phone, with a SIM and an eSIM, that could access the internet (away from WiFi). It did this via one of the SIM’s.

Now, if both SIM’s are active, it can’t access the internet via either SIM.

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Answer 8

Jan 30, 2025 12:26 PM in response to iYosuua

Assuming you actually mean the exact same problem:

When you turn one off there is only one line for cellular data, so it uses that one. If both lines are on you must select one for cellular data. The general instructions are in this: Using Dual SIM with an eSIM - Apple Support. Specifically:

One number at a time can use cellular data. To change which number uses cellular data, follow these steps:

Go to Settings.
Tap either Cellular or Mobile Data.
Tap Cellular Data.
Tap the number you want to use cellular data.

If you turn on Allow Cellular Data Switching, then while you're on a voice call on your voice-only number, that number automatically switches to use voice and data.6 This lets you use both voice and data while on the call.

If you turn off Allow Cellular Data Switching and you're active on a voice number that isn't your designated cellular-data number, then cellular data won't work while you're on the call.

To turn on Allow Cellular Data Switching, follow these steps: