@eleomir 

Since you are using AdGuard Home as a DNS server, it has nothing to do with the router if you have not blocked the DNS port,

what are the Upstream DNS servers on AdGuard Home? Is it set to the router or are you using a Public DNS server?

Are you using DoH or regular port 53, If you are using DoH what are you using as Bootstrap DNS servers? Is it a Public DNS or are you using the router?

You can also set Fallback DNS servers, what do you use there?

  @eleomir 

As MR.S states, if you use Adguard and some websites are blocked, and you cannot load them, consider asking the author of the block list. 

Upstream servers on ADG, if there are multiple, and you use parallel mode, that should not pose a problem, as the request is gonna be answered by all the servers and you use the fastest one. 

Thank you for the answers. In the Adguard config I'm using public DNSes as upstream - I have these set up, but I've experimented with different options when I run into these issues

https:// dns10 . quad9 . net / dns-query
208.67.222.222

(had to add spaces to the above address because of security validation on the forum)

Additionally I have 8.8.8.8 set as fallback upstream and use 1.1.1.1:53 as boostrap. 

I used to do parallel requests, but now I do load balancing, since I didn't see an improvement with parallel requests. 

This is also not about certain websites being blocked. Once a couple of weeks all of the internet goes down (or grinds to a crawl) - I can only access local devices. This only gets resolved if I restart the router/controller. 

I only have the router set as Private reverse DNS server, as that was the default config at Adguard. 

All of the above config is in ADH, the router only has the IP of the RPi running ADH set up as DNS address. 

Thank you for the help so far! 

  @eleomir 

With those settings in AGH, the clients skip the DNS function in the router, so the router can't be the problem. What hardware are you running ADH on? It should be Rpi4 or Rpi5. I've tried Rpi3 but it was way too slow.

  @MR.S I know that seems like the case, but what absolutely puzzles me is why only restarting the router resolves this problem. I can reboot the RPi/ADH and it does not help at all - I'm running it on a RPi 5 with 8gb ram and an SSD, so I don't think performance is a bottleneck. 

Net time this happens I will try to capture the request logs from ADH - I seem to recall that the DNS queries were not even reaching ADH, but I need to gather some proof there. 

  @eleomir 

yes it sounds strange, I run ADH on Rpi5 4GB along with lots of other applications and it runs like a rocket. so then we can rule out hardware problems.. I use an ER8411 as Upstream DNS servers on port53, then it goes on to NextDNS on DoH so I filter three times and have never had problems. You can try setting the router as Upstream DNS servers to test if it gets better or worse.

  @MR.S Thanks - I'll give it a go and see if there's any improvement in stability.