I used to run a Dual 150 with my WiFi only iPad but have been using an InReach for a year and a half or so. I had researched it prior to purchase and the answer is no. The InReach will not work without an active subscription.
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I'm not sure this is entirely accurate. It may be, just not sure. My InReach subscription is currently inactive. I use the subscription that lets you activate for a month at a time using which ever level that suits your upcoming needs.
I just paired my InReach with my iPad. The gps portion of the system appears fully functional, shows the current position on the InReach screen and is transmitting position information to the iPad as shown on the Earthmate app. Obviously the tracking and communication features aren't available.
Remember, InReach uses two different sets of satellites. The gps constellation (US government satellites) provides positioning information and doesn't require any kind of subscription for any gpsr device. The communication function of InReach uses Iridium satellites (commercial satellites) just like sat phones and that does require a subscription of some sort.
The reason I am unsure is my iPad has the gps receiver chip in it. Thus I can not be 100%. If I had a non-gps iPad to test with I'd be more confident. But having used Garmin handheld devices with bluetooth paired with non gps capable devices in the past, I am fairly confident the InReach will provide positioning data either with or without an active subscription.