as far as I know there's not a way to enforce "failover". When your router advertises DNS to the LAN, each LAN client discovers a number of DNS addresses... it's then up to them how they use it. One operating system may request from the first one, wait, and then fail over to the second one, but other operating systems will just send queries in parallel to all of them and take whatever comes back first
The easiest way is probably to just log into the router and change what is used on the WAN during the maintenance period for the PiHole