Having a problem using Bing Search on mobile broadband

I have a Bing API Search working when I connect my desktop to my landline based broadband, but not when I connect the same desktop to my mobile network based broadband?
It's exactly the same machine and code? The mobile broadband provider is three's accessed via a Huawei mobile router & it gives the error code 400 and response of "Our services aren't available right now". Because of my rural location I only get 1.2Mb on the landline, hence the use of a mobile router. I've developed many REST API interfaces to all sorts of providers and never had an issue.
Thoughts anyone? It suggests there's an issue with the endpoint, but I can find nothing in the documentation that suggests you have to adjust it for a mobile network.