Get a free firewall.
Personally, I recommend NoRoot Firewall, because it is simple and intuitive.
A much more advanced one is NetGuard.
I have both, they are small enough to have both on one device, although you can use only one at a time.
As I mentioned earlier, it is difficult to find an essential app that will allow you to disable it.
Android is set up so that it is near impossible to 'break' a device by disabling apps, because the ones that have to operate are not able to be disabled.
Rooting, of course, is another thing altogether- with that you can disable ANYTHING, and therefore make a device not able to work.
That is why rooting is made difficult to do for the average user.
This device isn't worth going that route, so leave that as a last ditch effort.
So far so good with what has happened so far, right?
Try NoRoot Firewall, and see how it works.
One nice feature is the log- if the notification rears its ugly head again, open the log and see which app is trying to make contact.
You can then block that app from any internet access, and even totally block the IP address that is trying to be contacted.
This could be very usefull in case the app cannot be disabled, or if it is grouped with other apps that you do not want to block internet access completely from.
Many apps are grouped together in this way, and NoRoot makes this obvious by grouping them together right from the start, so that you will automatically know.
(NetGuard does too, but not obviously, so NoRoot is much easier to use).
Another cool thing about NoRoot is that this IP blocking feature can also be used to block ads.
Any IP you block is blocked globally, meaning that no app can link to those addresses.
Although NetGuard has a nice ad-blocker, it works in a different way.
Even if this device will be basically camera and storage, you may still want to use it on Wi-Fi for sharing these videos and pictures, and even for browsing and other online things.
Let me also add a link here for an excellent browser, that is by far the fastest and most intuitive.
I have used it and derivatives of it for years, and it really is amazing.
You will probably want Lightning on your daily driver device as well.
Any operating questions about it, I have most of the answers, even answering questions for people on the devs page on GitHub.