Guys, I discovered the root cause for my issue. I have a Firewall hardware with Endian Firewall software for my whole home lan/wifi. From firewall logs I seen that it drops every broadcast packets coming from/to the Gateway. Broadcast messages is used by Gateway to discover its sensors, without these, it just go in timeout after 2,5 hours.
So I adjusted this behaviour and now only the broadcast packets sent to firewall is dropped, and the gateway never becomes unavailable again.
My advice is to check your router/modem/firewall and so on to leave wi-fi devices communicate each others without limitation.