@manjulapra 

I suspect devices on your guest network can't access DNS server.

To check that, connect device to Guest Network WiFi, check what it reports as its DNS server, try pinging DNS server IP address from that device. If I am right in my guess, that ping will fail.

Also, to confirm Internet access from Guest network, use same device to ping 8.8.8.8 - that should succeed.

  @Alexandre. Thank you. You are exactly right. How do I make it visible to the guest network?

I previously had PiHole DNS server IP address in IPv4 settings as the primary DNS server. Things worked fine but the PiHole sometimes stops working and then the Deco completely messes up the IP settings by changing the IP range. It is very hard to set an IP from the local IP range as the DNS IP.

Do you have a way I can open up the DNS server to the guest network?

TIA

  @David-TP Thanks a lot. I think that worked. Hopefully it'll work across my IoT devices and other settings too. If there's an issue I'll get back. 

Thanks again.