Hi Sunshineuk,
If you can't sign in, let's follow the guidance below to isolate signal from the issue:
"If you're not able to sign in using cellular data only
Make sure that your iPhone, iPad, or PC has a strong Internet connection and that you've turned on cellular data on your iPhone or iPad by going to Settings > Cellular or Mobile Data. If this setting is off, you might not be able to access your Apple ID and iCloud when not connected to a Wi-Fi network.
"
If you can't connect or sign in to iCloud - Apple Support
Also, restart the iPhone which can help a well: Restart your iPhone - Apple Support
Need more help, Get Support
Cheers.