I would put a client on the top of that hill that's solar powered. That way you can communicate with the cabin and with the river on the other side of the hill. You can set it to be an unmessageable node so that nobody tries to contact it directly.

The difference between a router and a repeater is that a router shows up in the list of nodes that exist, whereas a repeater does not. So it's probably better to use router instead of repeater on a normal basis.

The only way I would suggest using router mode is if that hill is the absolute highest point around for quite a long distance.

Even then though, I would suggest using client mode at first, and if there's anybody else in the area who has meshtastic coordinate with them to see if a router would make sense at the top of that hill.