Under MBIM mode, Sierra 7511/7565 modems successfully initialize and register with the network under 7b8, but it appears 7vB doesn't realize this happens, as the session uptime does not populate and the link does not pass traffic.

Interestingly under 7b8 serial mode, the 7511/7565 modem's usb serial ports are missing, so you cannot establish a connection to the modem in this mode at all.

Alternatively, the Sierra 7455 works just fine in MBIM mode in 7b8, and the serial ports work as well.

The MBIM logging is very cryptic, so unsure what's amiss here.

Under v7.1B1 I was able to get the Sierra 7511 to connect and pass traffic.