@David-TP 

I already do some tested but the issue remain same.

Test 1: Turn off smartphone WiFi. My smartphone should now be on voice/data from my cellular provider.

It is equivalent of you leaving my home and getting out of Deco WiFi range.

Test 2: Power off smartphone. It is equivalent of leaving the country and be in another country which does not have voice/data roaming agreements with my cellular provider.

Also i want to confirm that If I'm out of home, I never share our TP-link login credentials with my family to manage the Deco's from their mobile phones.

But sure, when I'm out of home for some days Deco's go offline with red light. So i suggest my family member to reset the Deco's and create a new TP-link account with their email and configure the Deco's with their phone. Once this process done, Deco's working fine but not solve the issue permanently.

I don't know, Why Deco network configuration follow the mobile phone that was used to set up the network. For this issues i have to left the phone in my home and then i go out, due to disturbance of network.

Please let me know a permanent solution for this.

All the best,

Shantanu