Are you aware of https://freenet-funk.de/ ? That's basically one GB per day for 99ct, no contract, only available per app, though no roaming, and no tethering, only for mobile devices, and using Telefonica/O². They got some bad press recently for kicking some users which overused it, and accusations of retroactively changing their terms of use. https://www.golem.de/news/funk-unlimited-freenet-kuendigt-ku... https://www.computerbase.de/2019-07/freenet-funk-datenverbra... https://www.computerbase.de/2019-07/freenet-funk-datenverbra... Anyways, maybe it fits your use case?