@rytterlund what image are you using? I think i did got it working with Resin OS 2.7.8+rev1but not with the versions before. But iam not sure.
And do you have a app container running? Maybe you can try to put it in Localmode. Then no app container is running. Because of the base img containers running on start the command: udevadm trigger and this is making the modem manger not to recognize the modem anymore. This is the problem iam running into.
So i think what you can do is to run localmode or run a really basic app. And then try if modem manger is recognizing the modem on the HostOS.