We went through the same thing at our hotels last year, so here is what I learned (I’m not an engineer, so this is the manager version).
Edge is software on the licensed gateway. It reads your Modbus devices on site, and each meter or controller shows up in Hermetry as its own child device. The gateway dials out over TLS, nothing on your site has to accept incoming connections.
If the broadband drops, licensed gateways can keep the readings locally and send them when the link is back. Each reading carries the time it was taken, so the history fills in at the right place. How much it can hold depends on the model, so ask for the figure for your model before you rely on it. Our integrator had to check that for us.
What surprised me was that alarm rules and flows run in Hermetry, on the cloud side, and the box only reads and buffers. So during an outage nobody gets an alarm e-mail, and when the backlog arrives the alarm rules look at the late readings by their own rules. Anything that must stop a press in a fault stays in the PLC or the controller, never in the cloud. Our maintenance lead was happy once he heard that.
The page that settled it for us: Licensed gateways and Hermetry Edge