There seems to be some known problem with the S9 and Android Auto.
It worked fine for me until the most recent Android updates.
Basically it seems to work fine, until you ask the system to send a text message for you.
You go through all the steps to send a text message, then just before your phone should send it you hear "This internet connection isn't strong enough at the moment" or something similar, but the connection is Unstable.
This is super frustrating and annoying, especially since it used to work fine and I used it all the time.
Supposedly Google is fixing it, but they have been saying it for months, and it still isnt fixed.
I wonder if it is a Samsung specific problem?
If so - can Samung please fix this!
See this link for more info, and add your $0.02 so Google takes more notice.