Unfortunately their plan is an IoT plan “Not Intended for Phones or Tablets” [0]

That’s exactly the issue - it’s a great plan, it’s just contractually stopped from being offered because a lot of people would potentially switch to that..! :)

To me, the fact that the restriction exists is a proof of the demand for this.

[0] https://embeddedworks.net/product/wsim0331-sub/