primary device changing from boot to boot

[Tim Harvey]

Greetings,

I've got a Sierra Wireless HL7588 modem which exposes three ttyACM
devs via the cdc_acm driver. The device appears to work fine with
ModemManager (1.6.8) but occasionally mm detects the primary device as
ttyACM2 instead of the usuall ttyACM0. In this case (detected primary
AT interface of ttyACM2), if I've already configured NetworkManager to
use /dev/ttyACM0 I can't bring up the modem.

What causes the primary interface to change and how am I supposed to
deal with it not being consistent with respect to NetworkManager
configuration?

Regards,

Tim