(No question; just wanted to report a working setup for USMobile as I've seen several questions about using them.)
This is using a USMobile WARP sim (not darkstar).
I had to set the APN to VZWInternet, it auto-detected something like VZWAdmin.
It got IP address with admin but would not route.
Once the APN was set it got both IPv4 & IPv6 (which is critical for us).
Firmware: 4.0 0800snapshot1
root@GL-X3000:~# ip addr show dev rmnet_mhi0
5: rmnet_mhi0: <NOARP,UP,LOWER_UP> mtu 1500 qdisc fq_codel state UP group default qlen 1000
link/none
inet 100.75.247.196/29 brd 100.75.xxx.xxx scope global rmnet_mhi0
valid_lft forever preferred_lft forever
inet6 2600:1007:b112:ede:1ae3:xxxx:xxxx:xxxx/64 scope global noprefixroute
valid_lft forever preferred_lft forever
inet6 2600:1007:b112:ede:6a89:xxxx:xxxx:xxxx/64 scope global dynamic mngtmpaddr stable-privacy
valid_lft forever preferred_lft forever
inet6 fe80::1ae3:xxxx:xxxx:xxxx/64 scope link stable-privacy
valid_lft forever preferred_lft forever
root@GL-X3000:~# ping www.google.com
PING www.google.com (2607:f8b0:4009:803::2004): 56 data bytes
64 bytes from 2607:f8b0:4009:803::2004: seq=0 ttl=115 time=31.211 ms
64 bytes from 2607:f8b0:4009:803::2004: seq=1 ttl=115 time=64.302 ms
64 bytes from 2607:f8b0:4009:803::2004: seq=2 ttl=115 time=60.388 ms
64 bytes from 2607:f8b0:4009:803::2004: seq=3 ttl=115 time=57.589 ms
64 bytes from 2607:f8b0:4009:803::2004: seq=4 ttl=115 time=39.537 ms