@Rob56 

It is a good idea to get UPS.

I can't speak for other brands, but APC line of UPS have had very idiotic design, where if APC UPS runs on its power and draw is non-zero but low, then UPS will shutdown itself after couple of minutes. With battery still almost at full capacity. 

Perhaps, they designed this when everyone had desktop PCs, but I was having just ISP router connected to powerful APC UPS. Imagine my suprise when I expected hours of uptime during long power outage, but UPS cut power to ISP Router in just under few minutes. Even bigger susprise for me was when I have found it is not a bug, but a feature. A feature that is badly documented and in many APC UPS models including mine can't be turned off.

Another quite idiotic design of that same APC brand is when power is restored, APC UPS may decide to run battery self-check which will briefly cut off power to everything connected to UPS.

Stay away from APC brand, especially older models with large battery.

Whatever UPS you choose, run practical test of having it fully charged, unplug and see how long it'll keep powering Deco X50DSL. You should be good if it runs for at least 15 minutes or whatever longest power outage duration you had plus few minutes extra.

Then, plug UPS back and watch it for about 5 minutes to see that it does not briefly cut power to Deco because UPS decided to run battery self-check when power was restored.