If Adguard is in use I believe option: -override DNS settings for all users- should be -off- for router, since all traffic goes through Adguard anyways. Adguard tab have 2 switches on it's page which should be in -ON- state, 1st is turning Adguard -on-, 2nd switch actually allow Adguard separate clients.
P.S. the only problem I see at the moment, my wifi mesh network connected to router doesn't allow Adguard separate traffic for clients on dashboard page (probably bcs all devices goes to internet through 1 main mesh device physically connected to the router). So I will test if Flint 2 can cover all my flat with proper signal strength and maby turn mesh network off.
P.P.S. It's only 2nd day I have this router, so there is plenty things to do, but I'm amazed about wireguard server option. Setup it in 15 second (only changed DNS server setting to one I need in standard configuration file). It works much faster than Tailscale (which is quite logical since do not need go through Tailscale servers 
) and also gives access to local network very easy.