Around us the nearest cell tower has battery backup for around 10 hours. The bigger problem is that when the power goes out, data usage spikes. Our coverage is iffy because of distance, and I’ve seen a 1-2mbps connection drop to 300 cps, like in the old, old days.
I take it the OP’s idea would be to install mwan3 on the mango, and when the main router goes down, the mango would switch to the cellular connection. That presumably would run off its own battery. If the battery bank was 20k mwh, then you might get 4 hours, yes?
At home my solution is less automatic. I enable the hotspot on the cell phone plugged into a battery bank. I might just think about tethering the mango in those circumstances as it might have more capabilities.