Hello @Cymro, thank you very much for posting in the community.

Cymro wrote

It rarely appears in the list of available WIFI connections, on my Iphone 13 mini or Ipad M4 Pro.

Are the iPhone 13 mini and iPad M4 Pro the only devices that fail to see the WiFi network of the WR3602BE? Or when this occurs, no client devices can see its WiFi network?

BTW, is it working on the default Router Mode? Do you have internet access when connecting a PC or laptop to its LAN port with an Ethernet cable?

What is the current firmware version of the WR3602BE? Please try resetting it to factory defaults and reconfigure it from scratch to see if you can connect to the WiFi network properly.

Cymro wrote

When it does appear, most of the time it doesn't connect.  When it does connect it normally disconnects within the first 90 seconds.

Please also try modifying the wireless settings on it, such as channel and channel width. Choose a clear channel for each band, and try changing 2.4G channel width to 20MHZ, 5G channel width to 20MHZ.

Cymro wrote

I've changed it from the default boost performance mode to balance, again without helping but making the connection speed very low

Could you please show me a picture of this setting for better understanding?