Welcome to the community and thanks for posting. I'm so sorry to hear you're not able to access the internet when using mobile data. This isn't something I like to hear especially coming from a new customer.
I'll happily help you out and keep it simple for you. Could you please answer the following questions:-
Are you able to make/receive calls?
Are you able to send/receive texts?
What is the make and model of the phone?
Did you get the handset from us or somewhere else?
What are you doing when you're getting the message? (Opening an app perhaps)
It may also be worth you getting the correct settings for our network once again. You can do this by selecting a handset from the list here and following the instructions.