I have turned off and turned on. I have deleted the app and reinstalled. I have had O2 replace the sim. I have upgraded my phone to an iPhone X. I have tried to check I have the latest version of WhatsApp loaded. My data roaming is on (other functions work with 4G). Any other bits of advice to share would be gratefully accepted. Thank you.
Check in settings if WhatsApp is allowed to use cellular (mobile) data @Crock
Also check if it is allowed to run in the background.
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