@ChrisBentley 

Hi,

Have you previously played around with the Parental Control settings on this router? If unsure, then check that the phone's Internet access isn't limited by a Parental Control rule.

Also, test what happens if you remove (or "forget") the Wi-Fi connection on your phone and then reconnect to the router's Wi-Fi network again. (you will be asked to enter the Wi-Fi password once more)

By the way, to get more time for troubleshooting the phone's Wi-Fi connection switch of "Mobile Data" on the phone. That should normally make the phone stay connected via the Wi-Fi connection and not switch back to 4G after 10 seconds.

  @woozle 

Hi,

Thank you for your advice.

Switching mobile data off seems to have done the trick, I've managed to maintain a constant connection all day. Glad to see that its not an router issue, though this never happened when I first installed the router so I don't know if I've accidentally tweaked a setting somewhere.

Thank you!

  @ChrisBentley 

I can't think of a router setting that would delay access to the Internet by 10 seconds or more.

In case you are using the default DNS servers of your internet service provider you could try if it makes any difference if you manually configure your router to use something like 8.8.8.8 for Google DNS or 1.1.1.1 for Cloudflare DNS instead.
(this can be done in the menu "Advanced" -> "Network" -> "Internet" -> "Advanced Settings")