Disabling your upstream DNS by unticking the 'Use dns advertised by peer' means you really should define your /routers/ DNS. Your dhcp will still hand out your AdGuard DNS via option 6.
Also you will need a defined dns so you get NTP updates.
It is one of the things I warn about in my thread about installing AGH on your router. I also include a separate WAN DNS script specifically for setting the router DNS to an external provider.
You shouldn't have a dns leak if you clients properly use your AGH dns and also use DNS interception to enforce that. Your router however doesn't need that protection as it will only been needed for updates and NTP.