I have an APC UPS for my 4g modem, router, and PoE switch. Since all three devices run off wall transformer blocks they are not particularly fussy about the quality of power they are given. With this in mind you are well able to use a "stepped or modified" sine wave UPS rather than the more expensive "pure" sine wave models.
You will find that your router and switch will use about 5-10W each so a UPS with a modest wattage or VA rating is perfectly good. All UPS suffer poor efficiency when very lightly loaded. Imagine a few low power devices fitted to a 1000W UPS - you will lose more power than it uses. Getting a UPS with just a few hundred VA rating will be fine for small devices.
One thing to consider is this... do all the devices run off a 12V transformer? If so, it is MUCH more efficient to get a 12V DC inverter which will plug into the mains to charge its batteries when the mains is present and deliver 12V DC to its outputs. You would then connect your router etc directly to the 12V DC inverter. Something to look into. Ask about this on any camper van/houseboat/RV forum and you will get more details.
As for the batteries in models such as the APC Back-UPS devices you should get a good five years of light use out of them before the batteries begin to degrade. For non-commercial equipment that does not need comprehensive warranties, once the original batteries fail a battery test you can readily replace them with generic 12V AGM batteries of the correct type from brands such as Yuasa.
There are lithium alternatives for a single device where you have a small LiFePO pack fed from a small lithium charger going into a 12V DC output board going into your device. Something like this... https://www.youtube.com/watch?v=IlEnEn7ptmc