Forget This Network not permanent (iOS)

I have two problems with iPhone and iPad WiFi in my home.

I use the 2.4GHz band for my iPhone because its range is better with the iPhone's small antenna, and 5GHz with the iPad with its bigger antenna for speed. I originally connected both devices to both bands, and once I decided on the above as optimal, I told the iPhone to forget the 5GHz band and the iPad to forget the 2.4GHz.

My first problem is that "Forget This Network" proves to be only temporary. After forgetting, it shows most of the fields as empty (DNS settings sometimes filled in but wrong) but once or twice a day after a period of disconnecting (aircraft mode, our in the street), I find it has connected to the other band and restored the password and other network settings from somewhere. I.e. sometimes the iPhone connects to 5GHz and iPad to 2.4, the reverse of what I want. I have to forget network and key in the password for the other band.

The second problem is that when this happens, the DNS setting is wrong. I usually have this set to Google's DNS - 8.8.8.8 and 8.8.4.4 but when the device connects to the wrong band, the DNS has reverted to past settings that I once used and which don't in fact work now.

How can I make both devices really forget completely the band I don't want to use for each? I don't want all the work of resetting the devices and rebuilding them from scratch, but I have tried 'Reset your Network Settings'. This cleared everything, but later, the old problem came back.

I thought it might be picking up old settings when connecting to iTunes, so I deleted both backups, did 'Reset your Network Settings' again, and backed up. But still it happens.

Can anyone help, please?

Devices: iPhone 5; iPad Pro 9.7 iOS10.0.2 on both.

I have a wired router and a WiFi Archer C9 router on the same subnet.