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.