I don't believe this is an issue with JMT nor the Triumph-2.
I believe this occurs because cellular providers still consider enabling the WiFi Hotspot on a phone to be an "option". In the US, this option is included in most or all plans, but perhaps elsewhere in the world it is a chargeable option. As a result, the carrier requires the phone to check with the cellular network every time the hotspot is turned on to make sure it is part of your plan. If you don't have cellular service, then the phone can't check to see if you have that option enabled.
With Android, you can use a Bluetooth connection between the Triumph-2 and the phone, but there are compatibility issues with Bluetooth between the iPhone and the Triumph-2 leaving you with needing a WiFi connection.
I would perhaps recommend getting an Android tablet (no need for cellular service on the tablet) and connecting it to your Triumph-2 by Bluetooth. When cellular data is available you can connect your Android tablet to the hotspot on your iPhone by WiFi and work as you are now.