Hey Gary,
Configuring Networking - Raspberry Pi Documentation
On the Raspberry Pi 3B+ and Raspberry Pi 4B, you will also need to set the country code, so that the 5GHz networking can choose the correct frequency bands. You can do this using the raspi-config application: select the ‘Localisation Options’ menu, then ‘Change Wi-Fi Country’. Alternatively, you can edit the wpa_supplicant.conf file and add the following. (Note: you need to replace ‘GB’ with the 2 letter ISO code of your country. See Wikipedia for a list of 2 letter ISO 3166-1 country codes.)
country=GB