An option for you. Setup a WireGuard server on your network and use the WireGuard mobile app to use that VPN connection when not connected to your home WiFi. Then you’re always using the internet through your pihole. That’s how I have mine setup and it works well.
I don’t think this accomplishes what he wants. The router DHCP will assign the second DNS address as you mention, but the devices will select one at random, not as a backup/failover. So what happens is that devices sometimes go through the Pi-hole and sometimes go through the secondary DNS address and receive ads. The only real way I’m aware of is to have a second pi-hole for redundancy. Personally, I decided to use a cloud based service (NextDNS) for this exact reason. I didn’t want my families internet to rely on devices that I host.