I have used the APC Smart-UPS line for my SO3 in the past, and it runs just fine. I live in the greater Seattle area, a very tech heavy area, so I can find the APC Smart-UPS 1500 on Offerup and Craigslist for $50-$100 that just need new batteries, so I buy them and put in new batteries. I have 4 or 5 around my house in use.
Here is a quick video of when I pulled the plug on the UPS with the router running. It’s a terrible video, because it was a spur of the moment thing to actually try it out. I was in the process of swapping out the router, so I just put a running router on the bed to simulate the load. Also, the one in the video is the Smart-UPS 1000. I have one of those.