While not specific to Dark Star, I'll add that US-Mobile Lightspeed (T-Mobile) network does work as I'm currently using a US Mobile Lightspeed physical sim in my X3000.
Also, it's my understanding that the TTL value that is input in the GUI is at the router level so to get a outgoing TTL of 64 you want to set the router to 65 and it will decrement to 64 at the modem. Perhaps GL-iNet can confirm this assumption.
Yeah the TTL value set in the GUI is the at the router level. As far as I know the US Mobile requires specific APN settings for different network. The issue might be more related to the APN configuration.