You can't do that with this device. It is exclusively a 3G/4G router. If you want to realise a fallback scenario, this would be possible with the ASUS RT-AC66U or a corresponding successor device. Here you can connect a 3G/4G modem (e.g. Huawei Stick E3372 (3G, 4G, GSM)) to the USB port and define a failure and fallback scenario between the USB port and the WAN via the firmware.

Personally, I consider the PWR-921 a bad buy anyway, because - at least in my case - it loses the connection every now and then. The connection then has to be re-established. If you are streaming something with Netflix, for example, the stream is guaranteed to break off. The connection is also lost quite frequently, sometimes even several times an hour.

Hello all
The fallback scenario works. I use it like this. The answer from Roowie08 is no longer correct.