Here is the solution to my original question (since some answers in here are just plain wrong):
You can have it act as client and AP at the same time, however, you are bound to using the same channel due to hardware limitations (unless you go back to 2.4GHz for either the client or AP side, which is just stupid).
To have the originally requested behavior (connect to an existing Wifi network and create a new Wifi network with a new SSID on a completely different channel) you need two independent physical radio chips in the device. Sadly there doesn’t seem to be a GL.iNet device that fits that requirement. If you need to have independent radio chips: MikroTik offers some routerboards with 3x miniPCIe slots you can populate yourself with network cards.