If the TLS client isn't a webbrowser, it doesn't have to use the list of trusted roots provided by the underlying system (be it OS or browser).

Certificate Pinning is the solution you'd look to if you have a clients that cannot securely ship with root certificates/cannot pick the root certificates they will use to validate a certificate when initiating a TLS connection.

Edit: I should add that I am not a mobile developer, so I don't actually know if the 'bring your own root CA' method is supported by the corresponding TLS libraries. But I know that this is possible 'in general'