Can't use mobile data on non-default sim, even if inactive when the second sim is open call
It all depend on your network strength and ability to handle voice + data.
Like 4G VoLTE has no issue about this. It can handle voice and mobile data simultaneously.
If you have other network, then make sure in your area network strength is high and your sim provider can handle both voice and data simultaneously.
Also mobile hardware and android os has features to handle voice + data.