The AT command port is always USB interface #3 on the EM7455 and most other current Sierra Wireless modems. So you can simply match against this attribute in sysfs. Either simple command line matching (this does not check the vendor id, so it could easily fail, but you get the idea):
bjorn@miraculix:~$ readlink /sys/bus/usb-serial/devices/ttyUSB*|sed -ne 's,.*3/tty,tty,p'
Or you could create a udev rule to help you identify the port, eg. by using a fixed symlink name:
Lots of possibilities. The clue is the fixed interface number.