@TP-Link Thank you very much for your reply.

Current primary DNS --> an internal server on my network (not 24x7 available) with bind9, which resolves internal request to internal machines and redirects remaining request to external DNSs 

Secondary DNS--> Google primary DNS 8.8.8.8

The problem is, when the primary server is powered down, KC110 doesn't reach KASA servers, nor can I access the cam from KASA app. KC110 led is blinking green all the time.

When the primary server is on, KC110 works fine

I also have another KASA device (HS110) on the same network which works perfectly fine with the primary DNS powered off (by using secondary DNS to resolve when primary gives timeout)

My main router is a Comtrend VG8050. This router provides Internet connectivity and DHCP server. I've also used other equipment as DHCP server on my internal LAN, but the KC110 behavior is the same

@dani1024 DHCP should be pushing out the address for the second DNS; is it configured to?  Simply having the secondary DNS available doesn't help unless DHCP tells devices where it is.

@Kirlin Yes it does.

It was a firmware bug, already fixed by TPLink

Thank you in any case.