Hi, I have a really simple dual wan failover setup.
Both ISP give dynamic IP, so I configured two dhcp client So far, so good. When ISP1 doesn't work, I just unplug cable and ISP2 takes priority.
Routes list and DNS changes automatically.

My problem is that when ISP1 restores its service, I plug in the cable and then
Routes list change, as ISP1 has distance=1 but...
DNS doesn't change. it keeps resolving on dns servers from ISP2.

In DNS list, I see both DNS servers from ISP2 at top of the list followed by DNS servers from ISP1.
This brings several problems like an IP doorbell stop working, weird issues on navigation...

any hint?

As you have multiple DNS servers added by having the DHCP client setting use-peer-dns=yes there is no control over which of the listed servers will be used. If the ISPs only permit DNS lookups from their servers via their connection you may well get lookup failures. The simplest fix is to set one or more non-ISP DNS provider (e.g. Google, quad9, Cloudflare, etc.) as servers under /ip dns and set the DHCP clients use-peer-dns=no

As you have multiple DNS servers added by having the DHCP client setting use-peer-dns=yes there is no control over which of the listed servers will be used. If the ISPs only permit DNS lookups from their servers via their connection you may well get lookup failures. The simplest fix is to set one or more non-ISP DNS provider (e.g. Google, quad9, Cloudflare, etc.) as servers under /ip dns and set the DHCP clients use-peer-dns=no

Thank you tdw!
Will try tomorrow, but I know it'll work.

viewtopic.php?t=182373 - See Para I.