With a device that small you could probably get away with a rather small UPS in terms of VA. I run a 2000VA powercool UPS myself (at current usage it lasts almost 45 mins on battery power), I personally think they only tend to catch fire if you neglect cleaning the vents and battery maintenance, (i.e drain battery's by 70% and recharge every few months or so, depending on brands instructions)

Connectivity wise though, multiple lines is the best option, including 4G wifi like you have. Landline with a 4G/5G backup could work well, unlikely for both to fail unless it's a city wide outage. (I have one of those a few months back)

This means we have... 2-4 witnesses running nodes from home as far as I'm aware. :D

Welcome to the club!

^{I'm a hive witness supporting the blockchain please consider voting for me.}