I live in a cellular dead-zone that is prone to frequent power outages so I am highly reliant on fiber Internet for both voice and data. My telecom backup power supply uses a discreet charger-battery-inverter setup. The battery in the middle is rated for 125AH at 12v and is a deep-cycle flooded lead acid (not AGM). It only powers my optical network terminator, firewall and POE wi-fi. It keeps me connected for days, not hours.

Sourcing a proper deep-cycle battery requires a bit of research.

Simply splicing a big-ass battery onto a consumer grade UPS won't work well, the little battery charger might not be able to fully recharge the new battery. The charge-discharge profile is all wrong.

Enterprise grade UPSs are not optimized for battery longevity, they are designed to recharge quickly and maximize runtime, both to the detriment of the batteries. APC in particular is notorious for overcharging batteries which can cause swelling and eventual rupture if they are not replaced on a regular basis. Also older APC "SmartUPS" would power off your load if you tried to use a regular serial cable to manage them. Surprise!

If you are looking for something robust for home use I suggest a reconditioned off-lease Eaton 5PX2000 with as many external battery packs as you want. They support network management, the front panel is actually useful, and they can be programmed to maximize battery life rather than runtime. They require a 20a 120v circuit but unless you are loading it up to more than 75% capacity you can get away with a regular 15a plug and an adapter.