Accroding tp to this link https://openwrt.org/docs/guide-user/network/wan/wwan/modemmanager It is under network section. But I could not find it at all. I did run ./scripts/feeds update -a && ./scripts/feeds install -a. didn't seem to help. Maybe it is under different name now? Thank you.
In menuconfig you can also use the search function with "Shift + 7" and search for modemmanager.
It's only available in master and not in 18.06!
But yes, it should be in Network according to:
Thank you very much for your help.