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

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

However, there is now a delay (1-10 min) in receiving most of the WhatsApp messages.  With the old 5S, when her friends sent her messages, they would see two grey ticks with 1-2 seconds.  Now with the 8 Plus, it could take up to several minutes before she sees the message (i.e. going from 1 grey tick to 2 grey ticks).  This was never an issue with her 5S.  She has similar issues with WeChat as well.

Nothing else has changed - it is the same home WiFi and her phone is in Airplane mode.  The new 8 plus is running the latest version of iOS, WhatsApp and WeChat.  The fact that this is happening for *both* Whatsapp and WeChat seems to suggest the new 8 Plus is the issue. We have already tried to "Reset Network Settings". I don't know if this is a hardware or software issue. Then this morning, I found that she couldn't access the internet with the 8 Plus, and we had to "Forget" our WiFi and set it up again in order to access the internet. I don't know if the two issues are related. I know that our WiFi is fine because my old 7 Plus was fine.

Does anyone have any suggestion? It is so frustrating because my previous iphone-5-to-iphone-7-plus upgrade was entirely problem free. I am considering returning the 8 Plus all together.