i think i found solution.. in my router settings there is there is switch called 'single SSID' (when enabled, router will broadcasts one single Wi-Fi network name (SSID) for both the 2.4 GHz and 5...

Camera is going offline after about 15-20 minutes of working correctly and will come back online after few minutes (time vary, can be 10 or 20 minutes). Camera is located about 2m from wifi router...