I agree that you should set up at least 2 Upstream DNS servers in AdGuard Home, which could be CloudFlare.'s primary DNS 1.1.1.1 and secondary DNS 1.0.0.1. On my AdGuard Home, I use DNS over HTTPS.
I also set up 2 separate AdGuard Home servers, in case one goes down due to problems or for maintenance. My primary AdGuard Home server runs in Docker on Synology NAS that is very reliable and my secondary AdGuard Home server runs on a standalone GL-MV1000W BrumeW that is connected via Ethernet LAN port only, without any WAN.
All I had to do to was to set up the DHCP server on my main router give out the 2 IP addresses as the DNS servers to clients. On GL.iNet/OpenWRT routers, you have to do that with a DHCP Option via LuCI → Network → Interfaces → LAN → Edit → DHCP Server → Advanced Settings → DHCP-Options.