@Laurence I'm not sure if or when that might come, but I'd encourage you to use the Send feedback option in the Stadia app to let Google know how eager you are to use your mobile data for Stadia.
In the meantime, I've got a few friends who have reported success at gaming over a mobile hotspot so that might be something to consider if you've got a spare device kicking around.
I tried this at work, I wasn't able to get passed the checking network spinner in the bottom left. Maybe you need 5G or something?