The latest version of Google Play Services comes with a feature that can ensure all your devices are always online. Austrian journalist Andreas Proschofsky has posted a screenshot on his Google+ account showing a new feature called "Instant Tethering." So long as you use one Google account for all your devices, you can program them to automatically create a hotspot connection to a phone with mobile data. Unfortunately, this is a limited rollout, so you might not be able to see the option even if you've already updated your phone or tablet.
According to Android Police, only Nexus and Pixel devices running Android Nougat 7.1.1 can access it for now. Further, both Pixel C and Nexus 9 tablets can only connect to a phone with data -- you can't use them as a WiFi hotspot, though they can take advantage of the feature even if you haven't upgraded since Marshmallow. Compatible phones like the Pixel XL can connect to each other's mobile internet with no issue unless your carrier blocks the feature. Since Instant Tethering will likely roll out to more devices in the future, make sure to check Play Services every now and then.
[Image credit: Andreas Proschofsky]