I have the same question. Is it really true that the cellular backup will not work on batteries? If this is the case, I would rather save on the modem, and invest in an internet router that accepts a SIM card for failover. It will cost me more in the end (new router, 4G modem, UPS), but it it will be a better solution. This way I can keep my entire internet up, not just the IRIS hub during the outages, so my other Wi-Fi devices stay alive as well (e.g. cameras).
The 4G modem is overpriced, by the way. In fact, the same model on Amazon costs about half, but IRIS explicilty says that we can only buy the version that they sell in the IRIS store.