Additional information...

On the most recent occasion, after my phone was unable to connect using the 2.4GHz and 5GHz bands, it was able to successfully connect a few minutes later using the 6GHz band.

  @RoryFan 

Hi,

Can you please check the number of "Total Clients" shown in the router's "DHCP Client List"?

Is it same (or just slightly higher) than the number of devices you have actually connected?

  @woozle Thanks for replying.

Currently, I have 17 clients connected according to the network map, and 18 showing in the DHCP client list.

You will note that these numbers are higher than the ones I was able to achieve earlier.  I recently (a couple of hours ago) enabled MU-MIMO to see if that changes anything.  This could be a coincidence, of course.

  @terziyski Absolutely, loads of space in the DHCP pool: 192.168.0.2 to 192.168.0.253.  I don't think lack of IP addresses is the problem 😅

Edit: No access control, parental controls or Homeshield features activated.  Subnet mask is 255.255.255.0.  Pretty much a vanilla setup.  After the first occurrence a couple of weeks ago, I rebooted the router, but obviously it's happened again.  This is not something one should expect from a top of the line router 😢

Edit: After the first occurrence, I implemented a weekly reboot (3am Monday), so it has only taken a little over 3 days for the problem to reoccur following the most recent reboot.