@terziyski 

Thank you for the quick reply. These are the settings I am using and seem to be the best options available.

@terziyski thanks for the suggestions. Yes the iPad Pro is the only device that won't connect to the 5Ghz network. It is running the latest iPadOS too. My Internet service provider  (1Ghz fibre to the premises connection) doesn't support IPv6 so I have that switched off on the Router. I am not using QOS which should be unnecessary on a 1Ghz connection; or any special features. If I reboot the router and/or reset the network setting on the iPad Pro it will sometimes connect but disconnects within an hour or two and then won't connect again. It is weird as the iPad has no problem connecting the 5GHz networks at other premises and connects fine to a tp-link RE450 access point. 

@Stenness OK, so after much experimentation I tried switching on Smart Connect and two days late the 2021 iPad Pro is still connected. I first thought that as with the 2.4 and 5Ghz channels split it would only connect to the 2.4Ghz with the Smart Connect. However, the download speed has consistently been 660 Mbs so it obviously is using the 5Ghz channel.

@terziyski Thanks, and yes it should but just didn't. The SSIDs I used were differenet xxxx-2G and xxxx5G but the passwords were the same. I am quite happy with the way it is working now but will take another look when iPadOS14.7 is released. With a new iPad Apple might not have got the wifi drivers just right. Though no problems with a tp-link RE450.

@terziyski thanks for the tip about tracking which band devices are connected to (I had to do this in the Advanced not Basic section) The iPad is definitely connected to 5Ghz. Indeed the only devices connected to 2.4Ghz are my Apple Watch, an Amazon Dot and a few Smart Home devices.