I know I’m pretty late replying but what you’re asking to do is actually possible. I’m having a similar issue where I’m casting my tablet to my chrome cast over the network but I’m using my mobile data to stream video to my tablet. That guy doesn’t know what he’s talking about. Mobile data always active if working correctly should function that way but it’s intermittent. There is an app called speedify that can accomplish this but it only gives you 2gb for free. Other recommendations I’ve seen were to reset your network settings. If I find a better solution I’ll update you if you’re still looking for a solution.
