iPhone 'Could Not Activate Cellular Data Network' Fix

What it means

'Could not activate cellular data network — PDP authentication failure' (sometimes 'You are not subscribed to a cellular data service') means the iPhone tried to switch on mobile data and the carrier rejected the connection.
The usual causes are a wrong APN (Access Point Name), an account that doesn't have a data plan or has run out of allowance, carrier-locked iPhone with the wrong SIM, or a carrier outage.
Calls and texts may still work; only the data side fails.

Affected Models

All iPhones with a cellular plan
Common right after changing carriers or activating a new prepaid SIM
Also common when travelling abroad with a local SIM that needs APN entered manually

Common Causes

Wrong APN configured for the carrier
Data plan ran out for the month, or prepaid balance is empty
Account hasn't been fully provisioned for data yet (new SIM, just activated)
iPhone is carrier-locked and you've inserted a different carrier's SIM
Roaming off when travelling abroad
Wi-Fi calling / VoLTE settings conflicting with the carrier's network
Recent iOS update reset cellular settings
Carrier outage in your area

How to Fix It

Toggle Cellular Data and airplane mode.

Settings > Cellular > Cellular Data > off, wait 10 seconds, on.
If still no luck, swipe down Control Centre, airplane mode on for 15 seconds, off.
This forces a fresh re-register on the carrier's network and is the fix for most one-off cases.
Check the account has data, and roaming is on if abroad.

Sign into your carrier's app or website to confirm the account has an active data plan and there's allowance left for the month.
If you're outside your home country, Settings > Cellular > [your line] > Cellular Data Options > Data Roaming on.
'Could not activate' frequently means the line is correct but data simply isn't enabled in your plan.
Set or reset the APN.

Settings > Cellular > [your line] > Cellular Data Network — fill in APN, Username, and Password from your carrier's website.
If there's no Cellular Data Network section (some carrier-locked iPhones hide it), the APN comes from a Carrier Settings update — see step 5.
A wrong APN is the single most common cause of PDP authentication failure on a new prepaid SIM.
Restart the iPhone.

Hold side + volume button, slide to power off, wait, hold side button to power on.
A clean reboot re-reads cellular settings.
Try data after; many APN-related issues clear here once the right values are in.
Check for a Carrier Settings update.

Settings > General > About — wait there a few seconds; if a Carrier Settings update is available, a dialog appears, tap Update.
Carrier-locked iPhones get APN settings via this update, so a missing or out-of-date carrier file is a common reason data can't activate.
If you've just inserted a SIM from a different carrier, the carrier update gives the iPhone the right APN for that network.
Reset network settings.

Settings > General > Transfer or Reset iPhone > Reset > Reset Network Settings.
You lose saved Wi-Fi passwords (re-enter them after) but no other data.
This clears stuck cellular state on the iPhone and often fixes 'Could not activate cellular data network' that survives other steps.
Call the carrier or try a different SIM.

If you've checked the plan, set the right APN, updated iOS, and reset network settings, ring the carrier from another phone — they can confirm provisioning, send a SIM replacement, or push a network refresh.
Or test a known-good SIM from a friend's plan in the same iPhone: if that works, the original SIM or the carrier account is the problem; if no SIM works in this iPhone, the modem may be faulty (a rare but possible hardware fault, common only after water damage or a hard drop).

Frequently Asked Questions

Why does the iPhone connect to Wi-Fi but won't activate cellular data?

Because Wi-Fi and cellular data are completely independent — Wi-Fi just needs a router with internet, while cellular data needs your iPhone to authenticate against the carrier's network with the right APN and an active data plan.
Three things cause most cellular failures: a wrong APN (especially on a prepaid SIM you set up yourself; look up the carrier's correct APN on their website and enter it in Settings > Cellular > Cellular Data Network), an account that has no active data plan or has used up the month's allowance, or a Carrier Settings update that hasn't installed.
Wi-Fi keeps working in all three cases, which is why the iPhone looks 'online' but data over cellular won't activate.