If your phones on a more recent version of android you need to do the manual hotspot thing as android doesn't let other apps change WiFi settings any more. You need to enable developer mode on android auto on your phone to be able to manually start the server. I haven't been able to get it to work automatically from detecting Bluetooth.
There's a wiki on line with the guides on.
Edit....
https://forum.xda-developers.com/t/android-4-1-headunit-reloaded-for-android-auto-with-wifi.3432348/
Method 2 on this
The only way I could get the free trial version working was with a USB otg cable, but the full version works great on WiFi.
I've had to set it up so that I manually start a hotspot on my phone, and start the android auto server on my phone as well, but hoping to automate that using an nfc tag in the car.
Fixed up the old tablet for the install - bought one with a broken screen, and swapped the motherboard to replace the one with the knackered USB slot.
It's got headunit reloaded installed on it so that it can pretend to be a car, and use android auto with it so that it is basically a bigger touch screen that controls the apps on the phone... Looks like it's going to work well.