Connecting an iPhone to a HomeKit enabled Router

Recently had to change my HomeKit Enabled Router (now Linksys MX4200) and whilst most HomeKit enabled devices automatically connected to the new Router three devices haven't. On trying to connect them I get the message "To add this accessory to your home connect your iPhone/iPad to your HomeKit enabled Router".

How do I do this please ? Both the iPhone and the iPad have wifi from the Router and are otherwise working well,