You are not alone. This is the post from Android Auto staff to solve the problem. Even it's not work with me, Hope it can help you, because it is free.
[FAQ] Can’t connect or stay connected - How to diagnose/address the issue - Android Auto Community
My solution was to buy an adapter. Actually i do not need a wireless connection, what i need is stable connection. Even disconnection few times after i use adapter, it is already better a lot than before.
Blow Link is where i bought it, it is work well. Amazon also have a lot.
American Express
The discount code is SMART15 which is i used successfully, after discount is $80+. I've asked them and every one is able to use the code.