Hello Cstanick,
still having problems with it. Tried with 3 different preplaced anchors. One on the paintcan, next to the Haze and in front of the house. Another one on the gardenlight in front of the oaktree and another in the Hedge, just right of the birdbath. The route you could take earlier through the hedge from the research center to the birdbath. They won't be recognised as a possible anchorpoint when i try to connect a zipline from another anchor. Even when i stand right next to it, there is nothing to interact with. No option appears to connect a zipline to them.
I play single-player on PC Steam and having this problem all the time. When i just start the game, reload the game or after severall hours.