This is een kwown issue @ShaunTan Simyo is working on a solution.

When running out of data you still become the warning SMS and that SMS is accurate.

Hoi @ShaunTan,

Thank you for your message.

What @J.D.L. said is correct. Thank you for your help!

There's a delay in processing your data usage in your My Simyo. We're currently working on solving this issue. The warning text however is still accurate and is being sent on time. 

Greetings,
Ashley

Dear Ashley and JDL

Thank you for your replies and assistance!

Hope the issue will be resolved soon.

Regards,

Shaun