Did almost exactly this for a gate camera at my uncle’s farm.
Cable: 100 m is per segment. Options, cheapest first: a PoE extender halfway (needs a spot to mount it, some are powered by the PoE itself), a small outdoor switch halfway with power, or two fibre media converters with a fibre run, which also kills any lightning worries between buildings. Kevin’s Wi-Fi idea works as a point-to-point bridge with two directional units, not with the camera’s own Wi-Fi.
Hermetry side, the refusal is expected. The cloud connects to the camera, so it has to be reachable at a public host name or IP, or through a Hermetry Edge on the site network (the platform then gets to it over Edge’s encrypted connection, no port forward). A private address like 192.168.x on its own is refused. Without an Edge box: forward the camera’s HTTP and RTSP ports on your router, allow only the platform’s addresses through the firewall, and add it with a view-only camera user, not admin. Then Devices > New device > Camera or recorder, test the connection, and put it on the board.
Check the stream codec. It’s H.264 only, no audio. A lot of new cameras default to H.265 and you’ll get a black tile. Live view stores nothing, it pulls video only while someone watches, and each minute somebody watches counts toward the plan’s camera minutes. If you put it on a wall screen all day that adds up. Limits on Camera limits and privacy.