Receiver shuts down every night at the same time

Call technical support and report a trouble.  They should perform a line quality test up to and including the gateway.  

If you want to speed things up in the 20 questions game, reboot your entire setup, including the WAP.   Do this before you call in so you can tell the support person the step has already been taken and the issue continues to persist.   Also, AT&T doesn't use a discrete router. It's called a gateway.  BGW210 for instance.