Can’t connect to network error

[ST6]

Hi I have turned phone on and off , reset all network settings, reset my iPhone X, updated software but keep getting the error cannot connect to network even though 3G shows. What else can I do as I am unable to use my phone currently unless on WiFi 

[MI5]

@ST6 

Where did you get the phone from?

Symptoms may indicate a barred handset.

IMEI can be checked here https://www.imei.info/

[ST6]

I upgraded my phone through o2. Thank you for the help I found a solution from an old post and changing the APN for mobile data in network settings to mobile.o2.co.uk which fixed the error

[MI5]

@ST6 

Unless you are abroad (which you didn't mention) you can use either apn, so I doubt that was the problem in your case.

Maybe just the fact that you reset the network settings in doing so fixed it.

You won't get visual voicemail or wifi calling unless you use the idata apn.