Thanks for your reply.
This might not be OpenWrt software related, but it's still related to hardware I use with OpenWrt. I'm on OpenWrt 19.07.3 r11063-85e04e9f46
The command seems to not be sufficient tho, I did get OK when doing AT+QCFG="usbnet",0, but the cdc-wdm0 device did not show up. Even after a reboot.
When I did a second try at AT+QCFG="usbnet",0 and it returned OK as the first time, but when I check it with AT+QCFG="usbnet"? it returns +QCFG: "usbnet",1. Is it possible that I've only got read access over the ttyUSB?