I'm having this problem (or something similar) on MM RR 5.7.x as well as Nougat Beanstalk 7.1. with various modems and have almost given up trying to fix it..
Any advice you can give would be appreciated..
Apart from collapsing the APN's (which didn't work).. I've tried different modems and tried switching between auth none and auth PAP as the Optus guides all show PAP.
Recent modems tried are XXUFOI1, XXUFPE2 and now back on XXUENA2 (which seems more stable).