My head unit had been dropping out occasionally and messages such as “your device cannot be recognised” or comments about needing to sync the phone time were appearing when using wired Android Auto; frustratingly this continued after installing a Carsifi unit a fortnight ago, though it always reset itself quickly.
This morning I did a reset on the head unit thanks to the suggestion above (single button press for ten seconds according to the manual rather than both buttons though), and after thirty seconds or so it rebooted, with Android Auto coming on a while later. So far, no issues whatsoever, and I’m now hoping for an experience as seamless as others seem to be having…