Hello LPimm,
Thanks for reaching out to the Apple Support Communities. We understand you are experiencing some issues using Siri with your iPhone while it is connected to cellular data. We are happy to help look in to this with you.
We would want to start by making sure that Siri is an available feature in your location. Take a look at the following article to see feature availability by region to ensure your region is covered:
iOS and iPadOS Feature Availability
We'd also like to ask some additional questions to get a better understanding of the situation.
- Are you able to use Safari to browse websites and use the internet while your iPhone is connected to a cellular data connection?
- When did you notice this issue start? Have you been able to use Siri in the past while not connected to a Wi-Fi network?
Keep us updated on what you find and the answers to those questions.
Best Regards.