How to setup AdGuard with a secondary DNS?

I’ve set up AdGuard on a Raspberry Pi in Home Assistant and I’ve configured my router to use it as the primary DNS.

However, the Raspberry Pi occasionally stops so I’ve set a secondary DNS in my router to 1.1.1.1 as a backup. But this means that as AdGuard doesn’t “find” the adverts the secondary DNS is used and so the ads are served.

Is there a way around this? Would Pi-Hole be better?