If your phone shows full signal bars but web pages won't load, apps can't connect, or everything just spins and times out, the cause is almost always one of a short list of things. Work through these in order — most people are fixed by step 2 or 3.
Check your APN settings first. This is the single most common cause, especially after switching carriers, inserting a new SIM, or getting an unlocked phone. Search your carrier's name to get the exact values — an incorrect or missing APN will block data even with full signal.
Confirm mobile data is actually turned on: Settings → Network & Internet (Android) or Settings → Cellular (iPhone) → make sure the toggle is on for your active SIM.
Toggle Airplane Mode on, wait 10 seconds, then turn it off. This forces your phone to reconnect to the network from scratch.
Restart your phone. This clears temporary network state that can get stuck after a software update or a weak signal area.
Remove and reinsert your SIM card (or reset your eSIM) — a SIM that's slightly loose or not fully seated causes exactly this symptom.
Check if you've hit your data plan limit or the account is unpaid/suspended — carriers often throttle to zero speed silently instead of showing an error.
As a last resort, reset network settings (Settings → General → Reset → Reset Network Settings on iPhone, or Settings → System → Reset options on Android). This wipes saved Wi-Fi passwords too, so only do this if nothing else worked.
New SIM cards almost always need the correct APN settings entered manually or downloaded automatically — until that happens, your phone has no way to know how to reach your carrier's data network.
This is usually a roaming APN issue. Some carriers use a different APN specifically for data roaming; check whether your carrier lists separate roaming values.