works on wifi but not on data" how to diagnose

TL;DR

When something works on wifi but not cellular, the app is fine and the network path is not. The usual culprits are the carrier's DNS, an IPv6-only path the server mishandles, a VPN or proxy active on data, or the phone blocking the app from using cellular. Test in this order: a different network, DNS, then device settings. You will find it in one of those three.

The query

"works on wifi but not on data" how to diagnose

Use this when

• Users report the app works on wifi but fails on cellular
• The reverse: works on cellular but not on a specific wifi
• Pages load partially or time out on one connection type
• You need to rule out the app before blaming the network

Not for

• General slowness on all connections
• Setting up a wifi router
• Carrier plan, billing, or coverage questions
• VPN product support

Steps

1. Confirm the pattern with a second network

Have the user test on a different wifi network and, if possible, a different carrier's data. If it fails on all cellular but works on all wifi, the pattern is real and it is the data path.

Expected output: confirmation that the connection type is the variable.

2. Check for a VPN, proxy, or private relay on the failing path

Phones often run a VPN or relay on cellular but not on wifi, or the reverse. Have the user disable it temporarily and retest.

Expected output: the VPN is ruled in or out in one test.

3. Test DNS on the failing connection

Carriers run their own DNS, and it misbehaves more often than anyone admits. Have the user switch to a public DNS in the phone's settings, then retest the app.

Expected output: you know if name resolution is the break.

4. Check the app's cellular data permission

Both major phone platforms let users block an app from using cellular data while leaving wifi allowed. This is the most embarrassing cause and the most common one. Check the app's data permission in system settings.

Expected output: permission confirmed on, or the ticket closes in ten seconds.

5. Look at IPv6 and carrier quirks

Some carriers are IPv6-only with translation, and some servers mishandle it. If everything else passes, check whether your service behaves differently over IPv6 and whether other users on the same carrier report it.

Expected output: a carrier-specific issue you can document or escalate with evidence.

Template: what to ask

Quick checks, [Name]:

1. Does it fail on cellular data but work on wifi every time?
2. Is a VPN or private relay turned on?
3. In your phone settings, is [App] allowed to use cellular data?
4. Which carrier are you on?

Variant phrasings

app only works on wifi

Start at step 4. The cellular permission toggle is the top cause.

site wont load on mobile data

Same playbook. Steps 2 and 3 cover most of these.

works on wifi but times out on 5g

Timeouts on data point at DNS or IPv6 translation. Steps 3 and 5.

Why it happens

Wifi and cellular are completely different network paths: different DNS, different IP versions, different middleboxes, different carrier policies. The app sends the same requests, but the network treats them differently. "Works on wifi" tells you the app and server are healthy, which narrows the whole problem to the path in between.

Edge cases

• Corporate or school wifi with a captive portal: the reverse pattern, where wifi is the broken path. Check for a login page intercept.
• Data saver modes: aggressive data savers can block background requests the app needs. Exempt the app and retest.
• eSIM vs physical SIM: provisioning quirks on eSIMs occasionally break data for specific apps. Rare, but worth asking about.
• Roaming: carriers route roaming data through the home network, adding failure points. Ask if they are traveling.

Provenance

Resolved from the public thread: https://vectle.com/posts/pstvn3sc5N5CBtukQpTBz1pQ

Resolved from the public thread: https://vectle.com/posts/pstvn3sc5N5CBtukQpTBz1pQ

Thread query: "works on wifi but not on data" how to diagnose

TL;DR: When something works on wifi but not cellular, the app is fine and the network path is not. The usual culprits are the carrier's DNS, an IPv6-only path the server mishandles, a VPN or proxy active on data, or the phone blocking the app from using cellular. Test in this order: a different network, DNS, then device settings. You will find it in one of those three.