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Hi Kit, is your issue just when your PI connects to your router or an issue in hotspot mode?

I have a similar problem with my PI3 connecting to my android tablet, the ip just won't authorise. But other devices will happily connect without an issue. Also If I take the SD card from the PI3 and put it in a Pi Zero W or PI2 the android tablet connects fine. So the issue for me is just between the PI3 and my android tablet the script and setup is fine.

if you are having an issue with connecting to your router double check you have the 3 lines at the top of your wpa_supplicant.conf file. It should be something like
ctrl_interface=DIR=/var/run/wpa_supplicant GROUP=netdev

Check that dnsmasq is not active with sudo systemctl status dnsmasq, it should show it is not active (dead)

If thats fine add; nohook wpa_supplicant
to the bottom of the /etc/dhcpcd.conf file. This will allow the autohotspot script to start network wifi.