RFs, you can probably still cover the bottom of the garden if you also get an additional sync unit, and place that next to the WiFi repeater, I’m thinking, from what you’ve said, in your conservatory.
If I’m imagining it right, this would get you as near as possible to the camera, with both a WiFi range extender, and a sync module. Be sure to use a separate WiFi ssid for the extender, so that it will be treated as a completely different system. If you use the same ssid as your main WiFi, you won’t be able to tell which one you are connecting to, and you could end up with it linked in to your main router, with all the range problems that would have.
To that new extended WiFi, with its own ssid, you would then setup a new sync module, which would obviously have good WiFi, it would be next to the extender. From there, add the spare camera to that sync module, and by default it will then be using the extender’s WiFi signal. Once set up, wander down to the bottom of the garden and try it out. If you have reasonable strength, and it seems to function in all regards, mount it as you finally want it.
The drawbacks of creating it as a separate system like this is that it shows under two headings in your app. One for each sync module, and you have to pick the chosen sync module first, before you can call up any cameras connected to that sync module. On the plus side, it’s all under the same account and all recordings are still in a single clip roll location, and are identified as to where they originated.
Look for spare sync unit on ebay or amazon, but be sure it’s suitable for the UK, if that’s your location. I’m thinking it is because you called it your garden.