Your colleague is half right. Buffering on licensed gateways covers the link going down: they keep readings locally and send them later with their original timestamps, so the history fills in. How much they keep depends on the model, worth asking about the specific one before relying on it. Licensed gateways and Hermetry Edge
But a buffer doesn’t help when the power is off. The gateway is off, and so are your meters’ RS-485 interfaces if they’re on the same supply. There’s nothing to read. So in your hut the UPS is what keeps data flowing, and the buffering is what covers LTE dropouts.
For the charger, I’d not build one. A DIN-rail DC UPS module (24 V or 12 V, takes the battery directly, does float charge and switchover, has a «mains fail» relay output) is about the price of the parts and the time you’d spend. If you do build, Rahul’s numbers are right: about 13.6-13.8 V float at 20 °C, current limited to around 0.1C (0.7 A for 7 Ah), and temperature compensation if the hut gets hot.
Wire the «mains fail» contact to a gateway input and send it as mains_ok. Then an alarm on mains_ok == 0 tells you about the outage itself, and you can make the offline alarm of the meters wait longer than the UPS runtime minus a margin. If you use the Remote sites ready solution, its UPS profile already has keys like ups_a_status and ups_a_charge you could fill.