Battery backup for Nokia 5G21 Gateway

Hello,

I am trying to figure out a battery backup for my Nokia 5G21 Internet Gateway. I have read several discussions here and am confused about which might be the best to get and how to correctly hook it up.

I am looking for one that will power my Gateway for several hours - not just a few minutes - but I am not sure how to judge the runtime based on the "size" of the backup.

I also want to be sure that my internet connection works when on the backup power- that's the whole point. I was told by CyberPower that the waveform is important and they say using a simulated sinewave might not allow for the internet to run smoothly.

Another question would be how to plug in the gateway. One discussion talked of using the 12VDC connector to power the gateway rather than just plugging the gateway into one of the surge plugs. Does that make more sense and would that allow for a longer run time?

Thanks for any help anyone might offer - since the gateway is my sole internet connection, I want to be able to use it during a power outage and want it to run for a length of time.