Hey, @Joseph47436. These steps should work with stretch just fine.
Since the Pi’s WiFi is 2.4GHz, of course the credentials should match that of a 2.4GHz network. WPA2 is fine.
The ssh and wpa_supplicant.conf files will be removed from the boot partition on the first boot - this is normal because these files are just a way for you to inject information into Raspbian. The information is copied to the relevant directory/file and then the source files in boot are removed. For instance, if you connects to a monitor and execute sudo cat /etc/wpa_supplicant/wpa_supplicant.conf you should see the credentials you entered into the boot partition copied to this location.
If your device is not connecting, make sure that port 22 is not blocked within your network and try connecting to pi@raspberrypi.local instead.
Sometimes connecting via hostname does not work for your network, and you’ll need a way to identify the IP of the Pi and connect directly to the IP. Generally you can connect via the hostname after connecting via IP the first time.
I hope this helps, let us know how you travel.
