Funnily enough, I (sort of) found a solution.
There’s a few things I learnt-
I’ve got 3 faucet timers, and it appears that only one can connect with Google Home at a time (ie. not all 3). Annoying, but that’s the way it seems to work.
Controlling the device doesn’t work on the iOS Google Home app at all. Every time I click on the device (all linked properly, as far as I can tell), it still consistently comes up telling me that it’s not responding, and the device is offline.
Having said all of that, I recently purchased myself a Google Home Hub, and much to my surprise, the voice controls work absolutely fine on here, and I’m now able to control the device with voice!
As to why the iOS app consistently tells me it’s offline, yet voice controls work on the Google Home Hub, I’ve got no idea. Clearly the connection is fine. Interestingly, the voice controls don’t work on the iOS app either.
Hopefully that helps a little?