Delay in receiving Whatsapp messages after iPhone 5S to iPhone 8 Plus upgrade (Wechat also)?

I just upgraded from iPhone 5S to 8 Plus. I used the Backup and Restore options in iTunes to transfer my apps and data, and after transferring the SIM card to the 8 Plus my phone was good to go.

However, there's now a delay (1-10 min) in receiving most of my Whatsapp messages. With the 5S, when my friends sent me messages, they would see two grey ticks with 1-2 seconds. Now with the 8 Plus, it could take up to several minutes before I see the message (i.e. Going from 1 grey tick to 2 grey ticks). This was never an issue with the 5S.

Nothing else has changed - I'm using the same Wi-Fi at home and my phone is in Airplane mode. The new 8 plus is running the latest version of iOS and Whatsapp. Also, this appears not to be a Whatsapp-only problem because I'm experiencing the same kind of delay with WeChat as well. It almost seems like some setting in iPhone is holding up the messages. I have already tried to "Forget" my Wi-Fi network and re-establish it.

Has anyone experienced this before?

Delete and reinstall these apps and they will work better.

After trying all kinds of solutions - uninstalling/reinstalling the two apps, restarting the phone, resetting network settings, etc - finally found one that works:

Settings → General → Reset → Reset All Settings

Now everything is finally back to normal.

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