iOS 27 update mobile network issues

What you describe points to a network/modem-related problem rather than simply an iOS update.

Possible causes include:

• Poor or unstable cellular signal: The modem may work harder when the signal is weak, increasing heat and causing temporary disconnections.
• 5G ↔ 4G/LTE switching: Repeated network handovers can make the modem work harder and occasionally interrupt connectivity.
• Carrier/network issue: A problem with the SIM/eSIM, carrier provisioning, or local network can produce repeated dropouts.
• iOS/modem software issue: A software bug can affect both cellular performance and thermal behaviour.
• Hardware issue: If this happens consistently across different locations and networks, the cellular modem/antenna or related hardware becomes more suspect.
• A particular app: If the problem occurs only with certain apps, background data activity or an app malfunction could be contributing.

A useful diagnostic test: try these one at a time:

1. Switch from 5G to LTE/4G
   1. Settings → Cellular/Mobile Service → Cellular Data Options → Voice & Data → LTE
   2. Use the phone normally for several hours.
2. Compare Wi-Fi and mobile data
   1. If the iPhone remains cool and stable on Wi-Fi but heats up and loses connectivity on cellular data, that strongly points toward the cellular modem/network/SIM/carrier path.
3. Try another SIM/eSIM
   1. If possible, test another carrier's SIM in the iPhone.
   2. Conversely, test your current SIM in another phone.
4. Observe the signal when the failure occurs
   1. Note whether the signal bars suddenly disappear, change to SOS, or remain normal while data stops working.
5. Restart the iPhone and check iOS/carrier settings
   1. Also check Settings → General → About for a possible Carrier Settings Update.

If the phone becomes noticeably hot at the same moment the cellular connection disappears, especially if this happens repeatedly regardless of location or carrier, I would have Apple run hardware diagnostics rather than treating it simply as an app or battery issue.

This could easily be a bug introduced in the last upgrade. A portion of the modem firmware and software is written by the carriers. There are already community members posting about fixes from their carriers.

I personally faced a similar issue last year with my iPhone 17 Pro Max and AT&T. They tried an OTA reprovisioning and it repeatedly failed. I went into an AT&T store and issue resolved in about 10 minutes.

You can check for any OTA updates by following this path,

iPhone > Settings > General > About > if an update is available from the carrier a notification will appear

Typically the next update from Apple is 4 to 6 weeks after the initial release. I’d talk to my carrier.

I had the same issue after updating to iOS 27. Found the FIX. It’s the VPN on your phone. Turn off your VPN or Delete your VPN app. Works perfectly fine after. Then you can reinstall the VPN app.

To fix dual-SIM network dropouts after updating to iOS 27, follow these troubleshooting steps:

1. Disable and Reinstall VPN

Go to Settings > General > VPN & Device Management and completely disconnect or delete any active VPN profiles. Alternatively, remove the VPN app entirely, restart your phone, and then reinstall it.

Verification: Check if cellular data remains stable with both SIMs active after disabling the VPN.

2. Switch Cellular Voice & Data to LTE

Go to Settings > Cellular (or Mobile Service) > Cellular Data Options > Voice & Data and select LTE instead of 5G.

Verification: Ensure your signal bars remain steady and data works consistently on both lines without dropping.

3. Check for Carrier Settings Updates

Go to Settings > General > About and wait for 30–60 seconds. If an update is available, a prompt will appear to install it. If issues persist, contact your carrier to have your physical SIM or eSIM reprovisioned.

Verification: Confirm that your carrier version number matches the latest release and network stability returns.

GOT THE SOLUTION

general>mobile services > voice and data> select LTE or any other options

boom it works 😭

AT&T released an update late yesterday and is pushing it to users. Please go to Settings > General > About > and wait a minute or two for a notification to appear. If it does, follow onscreen instructions to install. A restart will likely occur. You might want to check with an AT&T corporate store if it’s not been pushed to you yet.

If you're using a VPN, disable it and check if the error goes away. Go to Settings > General > VPN & Device Management and turn off any active VPNs. Disabling a VPN only from the iPhone settings may not completely turn it off. To fully disable it, you need to switch it off from the VPN service provider’s app or remove the configuration profile they installed. Only then is the VPN truly off, which can help if it’s causing issues with cellular or Wi-Fi connectivity.

Do not use a VPN

Ensure the VPN is fully turned off, regardless of whether you are connected. At times deleting the VPN profile may be necessary.

Disable VPN

VPN or proxy connections might prevent your device from contacting the update servers.

Contact the VPN Service provider and disable it for the time being. In some cases, if the VPN server or network is configured to block certain domains or IP addresses, it might inadvertently block Apple's update servers. This could prevent your device from connecting to Apple's servers to download the update. Make sure that the VPN settings do not block Apple-related services.

GOT THE SOLUTION

general>mobile services > voice and data> select LTE or any other options

boom it works 😭

I had the same issue. Found the FIX. It’s the VPN on your phone. Turn off your VPN or Delete your VPN app. Works perfectly fine after. Then you can reinstall the VPN app.

Have you contacted your cellular service provider? In my experience most network issues are resolved through the carrier. Your SIM card(s) may need to be reprovisioned. Please contact your cellular service provider’s support team and see what they say.

Did AT&T reprovision you eSIM? My son had this issue today and just like last year AT&T reprovision his sim and he was good to go.

I just found a fix. Delete your VPN app, and re install again

Delete your VPN app and reinstall it again. This fix worked for me.

Delete your VPN app and reinstall it again. This fix worked for me.

... both are new and actual ESims in my case.

And if only one line is active both works without any problem. Just in case you activate both lines the problem exist. In my opinion this is an IOS issue and not a carrier issue!