Seeing "No Service" or a missing signal bar on your Xiaomi 15 can stop you from making calls or using mobile data. It's a common issue with Android phones, and on the Xiaomi 15 running HyperOS 2, there are a few specific things to check.
I'd start with the quick fixes. They solve the problem more often than you'd think.
Restart Your Phone and Check Airplane Mode
A simple restart clears out temporary software glitches that can mess with the radio. Just press and hold the power button, then tap 'Restart'. While you're at it, pull down the notification shade and make sure the airplane icon isn't highlighted.
If Airplane Mode is on, tap it to turn it off. You can also go to Settings > Connection & sharing to verify it's disabled there.
Reinsert Your SIM Card
A loose or dirty SIM card is a classic culprit. Grab the SIM ejector tool that came with your phone and pop out the tray.
Take out the SIM card, give the gold contacts a gentle wipe with a soft, dry cloth, and then reseat it firmly in the tray. Push the tray back in until it clicks. Wait a minute for the phone to search for the network again.
Toggle Mobile Data and Check Signal
Sometimes the mobile data connection just needs a nudge. Swipe down twice from the top of your screen to open the full Quick Settings panel and tap the mobile data icon to turn it off, then on again.
Also, take a look at your signal strength. If you're in a basement, a thick-walled building, or a rural area, you might simply be in a dead zone. Try moving near a window or to a different location.
Update Your Access Point Name (APN) Settings
Incorrect APN settings mean your phone can't connect to your carrier's data network properly. These can sometimes get corrupted after a software update.
Go to Settings > Mobile network > SIM cards & mobile networks. Tap on your active SIM, then select Access Point Names (APN).
If the settings look wrong or are blank, you'll need to enter them manually. Your carrier's website has the exact APN details you need. After entering them, save the new APN and select it as your active profile.
Reset Your Network Settings
This is a more thorough step that resets all network-related configurations back to factory defaults. It will erase your saved Wi-Fi passwords, Bluetooth pairings, and VPN settings, but it won't touch your personal photos or apps.
To do this, navigate to Settings > About phone > Factory reset. Here, tap on Reset network settings and confirm. Your phone will reboot and you'll need to reconnect to Wi-Fi networks afterward.
Check for a Carrier Service Outage
Before you dig too deep into your phone, it's worth checking if the problem is on your carrier's end. Network outages happen during maintenance or due to technical issues.
You can visit your carrier's website on a Wi-Fi connection, check their social media accounts, or use a site like Downdetector. If it's a widespread outage, you'll just have to wait it out.
Use the Phone Information Menu
Android has a hidden diagnostic menu that lets you check the radio status. Open your phone dialer and enter *#*#4636#*#*. This should open the "Testing" menu.
Tap on Phone information. Here, you can see your signal strength in dBm, check your network type, and even run a ping test. You can also try tapping the three-dot menu and selecting Run Ping Test.
If you're feeling adventurous, you can toggle the radio off and then back on from this screen, but a phone restart does the same thing more cleanly.
Disable Any Active VPN
If you use a VPN app for privacy or work, it could be interfering with your cellular connection. A misconfigured or faulty VPN can route all your traffic, including cellular data, through a server that's down or slow.
Open your VPN app and disconnect from it. For a more complete check, go to Settings > VPN and make sure no VPN profile is set to "Always-on". You can temporarily uninstall the VPN app to see if it resolves the signal issue.
Update Your Xiaomi 15's Software
HyperOS updates frequently include bug fixes and improvements for network connectivity and stability. It's always a good idea to be on the latest version.
Go to Settings > About phone > HyperOS version. Tap on the "Check for updates" button. If an update is available, download and install it over a stable Wi-Fi connection. Your phone will restart as part of the process.
Try Your SIM Card in Another Phone
This test tells you if the problem is with your Xiaomi 15 or with the SIM card itself. Put your SIM into another unlocked phone (an old one is fine).
If the other phone also shows no service, the issue is with the SIM or your carrier account. You should contact your carrier's support. They can check your account status and send you a replacement SIM if yours is faulty.
If the SIM works perfectly in another phone, then the issue is likely with your Xiaomi 15's software or, less commonly, its hardware.
Perform a Network Operator Search
You can manually force your phone to search for and register on your carrier's network again. Head to Settings > Mobile network > SIM cards & mobile networks.
Tap on your SIM, then look for Network operators or a similar option. Tap it, and after a short scan, a list of available networks will appear. Select your carrier's name from the list instead of leaving it on "Choose automatically".