avast detected a weak password on the router.
That doesn’t mean it is a WiFi password.
The router itself has (or at least should have) a password on it.
Next to that a router nowadays has a OS.
That can also have a weak password.
Since avast is not making changes to the router, I doubt it is avast that is causing it to malfunction.
Already did a hard reset on the router ?
What OS/SP ?
What exact version of avast ?
Does disabling one of the shields makes a difference ?
Already tried a repair of avast ?
Already performed a clean installation of avast ?
Are you getting a error when trying to use that router ?
More information is needed to troubleshoot.