I tried with modemmanager on omnia with EC20 modem but I had the same issue above. Even though I had correctly specified the sysfs path.
From my experience with omnia, for the moment, I would suggest to use qmi cellular protocol to set up the modem and test it. In Luci then you can insert the pin code. If this works then you could test with MBIM. I expect that working Luci for modemmanager will be available after the next openwrt release
ps: consult this also if you will use qmi