There are places and situations where people are struggling with frequent power outages causing interruption in the internet access, this leads to dropped calls, dropped download sessions etc.
Typically, these power outages are of short duration as backup generator takes over the load within few minutes. A short disconnection of power causes reboot of broadband wifi routers and re-connection of internet for all the devices on the network.
To tackle this problem of short power outage, i found this interesting cable on aliexpress sold for 1Euro including shipping.
The black plastic housing towards the usb end contains a booster pcb(MT3068 converter) as shown here
As shown in the following picture, you can connect 5v Dc adapter to the battery bank's charging port and 5v output of batterybank powers the broadband wifi router through this booster cable.
Backup time of such a solution depends on capacity of the battery bank. In your situation if the power outage is of short duration, then you can manage with lower capacity battery bank, else you can go for a higher capacity one.