The analyser app showed that there was 90% interference on Channel 11 and suggested I use channels 3, 4 or 5 however these were not available on the router.
I switched the router from Smart to channel 1.
The analyser is now showing that the router is on Channel 36 which is "optimal". No interference is showing on the app.
I'll leave these settings for a couple of days and see of things improve.