Thanks so much for your help in alerting the Unifi TV team.
At first TM emailed me saying that the issue was resolved under report Number etc. But later I emailed back saying the issue still persist. The next day, TM Technical Support came to my door and fix the problem. It has been 5 days, no more error code 26000. So I'll share here what he did:-
1. Go to Settings at the front page of Unifi TV
2. Under Network Settings, Enable WLAN
3. Choose unifi network & Enter Password
4. Reboot system
According to the Technician, the unifi box (white one) has to be connected/linked this way even though WLAN cable is plugged in, if not Error code 26000 will appear.
There goes my 7 months problem solved. Hope this would also benefits others