BACK TO BASICS…
Putting the KODI config via backup/restore aside and concentrating on the basic setup from scratch:
I now have two SDcards for the new system, one created using the “Raspberry Pi Imager” application and one created from a download of the “installer” disk image from OSMC and written to SDcard using Balena Etcher on Windows. Both are the most recent OSMC (June 2023).
Both start up OK and perform some initial tasks before showing the KODI interface screen.
And on both of these the WiFi connection from MyOSMC network config page fails in the same way.
Note that WiFi must be coming up because it lists all of the WiFi hotspots in the vicinity so I can select mine from that list.
But then after I have been asked to enter the password it fails to connect in both cases. 
My network is served by an ASUS RT-AC86U router with both 2.4 and 5 GHz radio.
On this my old OSMC boxes run just fine on WiFi using RPi3B and RPi3B+ from about 6 years back.
But my new RPi4B, which connects fine using PiOS rather than OSMC, will not connect to WiFi with OSMC.
Is this caused by a quirk in the OSMC f/w for RPi4B regarding the WiFi network???
If so is there a solution or should I just drop the whole upgrade idea?
I have googled the problem and it seems like I am not alone here…
These threads describe the problem too:
https://discourse.osmc.tv/t/wifi-connection-on-r-pi-4/93237
https://discourse.osmc.tv/t/august-update-and-wifi-channels-above-100-not-working/89996
And the solution would seem to be:
sudo apt update
sudo apt install iw crda
iw reg get
#No real output
sudo iw reg set SE <= My region should be SE?
iw reg get
#Now SE is shown as the region, so make persistent:
sudo nano /etc/default/crda
#Add the correct domain:
REGDOMAIN=SE
Then reboot and try connecting.
But unfortunately I am STILL getting the fail to connect error message.
I have now also tried to connect to my guest network on 2.4 GHz but the exact same happens.
But I see this:
$ iw reg get
global
country SE: DFS-ETSI
(2400 - 2483 @ 40), (N/A, 20), (N/A)
(5150 - 5250 @ 80), (N/A, 23), (N/A), NO-OUTDOOR, AUTO-BW
(5250 - 5350 @ 80), (N/A, 20), (0 ms), NO-OUTDOOR, DFS, AUTO-BW
(5470 - 5725 @ 160), (N/A, 26), (0 ms), DFS
(5725 - 5875 @ 80), (N/A, 13), (N/A)
(57000 - 66000 @ 2160), (N/A, 40), (N/A)
phy#0
country 99: DFS-UNSET
(2402 - 2482 @ 40), (6, 20), (N/A)
(2474 - 2494 @ 20), (6, 20), (N/A)
(5140 - 5360 @ 160), (6, 20), (N/A)
(5460 - 5860 @ 160), (6, 20), (N/A)
Should it not work at this point???`??