Why does the device show as a phone on T-Mo networks?

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@Minker17

What data is it reporting back?

Depends on what TTL you have set. Generally a TTL of 65 will use Mobile Phone data on a mobile phone plan.

@Minker17

Are there other manufacturers (not GLiNet) that make mobile routers that report themselves the same way?

No, Only T-mobile lets you use any of their plans on any device (Except the Home Internet Plan), other Providers will require you to use an authorized device from them, for the plan you purchased with them. You can however tether your Mobile Phone via USB (If you have it on your phone plan) to a GL.inet router and set the TTL to 65 and use the unlimited data from you plan.