Data problems

If you are using Smarty for work, it may be that any VPN you are using to connect to your workplace is blocking the connection.

Also, assuming that you have used Smarty successfully for work purposes in the recent past, you may have fallen foul of their terms and conditions :

Section 5.15 & 5.16 of SMARTY's terms & conditions state: 

...
This SIM card should not be used as a permanent long-term alternative to home broadband. If we determine in our sole opinion that your use of the SMARTY Services is in breach of these Terms and/or exceeds that reasonably expected of someone using the Service for legitimate personal, noncommercial use, then we may at our discretion terminate (in accordance with Section 9), or Suspend (in accordance with Section 8), or  restrict your SMARTY Service. As an example, without limitation, we consider that if you regularly tether more than 8 devices that this is unlikely to be legitimate personal, non-commercial use.

I've had the same problem since switching to Smarty last week & have finally found the cause to be my "Google 1 vpn". Immediately after disabling it my connection is perfect.

Perhaps there's a network fault at your location, or your SIM is damaged or faulty ELIZABET_125933.

You can check the current network status online at: 
https://www.three.co.uk/support/network_and_coverage/network_support