Google's help page for the new feature says that it's been certified to work with the following tablets: Nexus 7 - K009 (US LTE), Nexus 9 - 0P82300 (US LTE), iPad Air 2 - Model A1567, iPad mini 4 - Model A1550 and Galaxy Tab S - Model SM-T807V. Google says that other tablets and even phones can also be used provided they support the T-Mobile network.
Phones with a data only SIM will not be able to make calls or send texts using the cellular network although using an OTT app like Hangouts should work. Using your tablet or phone as a hotspot is also not supported with data-only devices which is surprising as it is allowed with Project Fi phones. Presumably that means you can't add a mobile hotspot. I wonder if cellular capable laptops will be allowed?
At this time, you must have a Project Fi phone account to be able to add a data-only device. Hopefully Google will eventually open data-only device eligibility to users without a Project Fi phone.
Project Fi is invite only and a Nexus 5X, 6 or 6P phone is required. Project Fi on a phone costs $20/month plus taxes for unlimited talk and SMS. Data is available in 1 GB increments priced at $10 per GB. If you don't use all your data Google will give you a credit of approximately 1¢/MB toward your next month's bill for the unused amount. Data-only devices share a Project Fi share the user's Project Fi phone's data bucket.
Unlike Project Fi phones, which switch between the Sprint and T-Mobile networks depending on which one has the best signal, data-only devices are limited to T-Mobile's network.
Sources Google (1), (2), Android Police
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