I have an Blackview BV 8000.
Some Apps like Chrome, Stack exchange, Facebook, WhatsApp... Work without a problem.
However apps like Gmail, YouTube and Google play store don't work on cellular data. I always get an error message, that tells me to check my internet connection (cellular or wlan/wifi, that I know is good).
In settings there is a network access control panel, where I unlocked those apps, however they still don't work under cellular data.
I realize this is a pretty basic answer, and you might have already tried this, but have you tried uninstalling and reinstalling those apps? Even if it's saying "check your internet connection", sometimes error messages don't reflect the real problem. Those apps you mentioned (Gmail, YouTube, and Google Play Store) all come from the same developer. If this problem just started recently, it could be a glitch in a recent update.
Alternatively, are you sure that you're logged in to those apps? The apps you mentioned are all dependent on your being properly signed in to your main Google account, via your device's accounts settings. If you've inadvertently logged out or deleted your Google account login, you'll lose access to those accounts.
Of course, if you haven't done this already, try restarting your device (turn it on and off). No joke, that actually fixes a lot of problems. :-)
