A normal UPS converts DC -> AC and then your devices will convert AC -> DC. This conversion is extremely inefficient with small loads. Most UPS units don't even advertise their efficiency below 50W.

You're on the right track with a DC Mini UPS. There are a handful of units on Amazon and all of them seem to be really bad.

The only reliable option I have found so far is from a company called Precision Group aka Precision Power:

https://goprecisiongroup.com/product-category/micro-ups/

These units are used by fiber ISPs (Ziply, CenturyLink, Frontier) to keep the phone line alive to ensure people can dial 911.

I bought a few of their PP75L-12P units on eBay. The PP75L-12P can handle 75W and often comes with two battery packs. Up to four battery packs can be stacked on one unit. Look for ones that come with a 5.5mmx2.1mm Y cable e.g. PP7P5521-Y-3.

Example eBay unit (not an endorsement of seller nor item, YMMV):

https://www.ebay.com/itm/185758336880

I have an ASUS RT-AX88U router with the same 19.5V 2.31A DC 5.5mmx2.5mm barrel adapter. This DC to DC adapter can accept 12V and output 19V (which is within 5% tolerance) using the N07 tip:

https://www.amazon.com/dp/B00PW4IMVS

The adapter's input is 5.5mmx2.5mm and the output of the Mini UPS is 5.5mmx2.1mm so you'll need a DC barrel converter:

https://www.amazon.com/dp/B07MC7TKJ2

Most of these Mini UPS units come with a 7 pin 2.5mm pitch phoenix contact connector that you can screw wire leads directly into if you want to make your own DC breakout cable.

The PP75L-12P with two battery packs and the 12V to 19V adapter powers my ASUS RT-AX88U for 11-12 hours.

You can avoid the 12V to 19V conversion entirely if you buy an ASUS RT-AXE7800 as it supports 12V 3A input:

https://www.asus.com/networking-iot-servers/wifi-routers/asus-wifi-routers/rt-axe7800/techspec/

The MB8611 has a 12V 1.5A (18W) adapter but average power consumption is likely less:

https://motorolacable.com/documents/MB8611-DataSheet-21920.pdf

Enjoy the scavenger hunt! :)