ok.. I rephrase from the beginning

My goal: to set fix IP address for my kids pc
My reason: to set rules (parental monitoring) on this IP and to map my kid's HDD on my pc
My problem: My kids IP keep changing
My solution: reserved the same IP (currently on dynamic mod) to my kids MAC address pc. Otherwise the monitored IP might change to my wife's pc and defeat the idea of monitoring my kid's pc activities

TL-WR1043ND problem:
My step: follow the unifi setup provided by the manual. All pcs are connected to LAN and have access to internet
My question: Upon checking the note under Network LAN help note no.2
This note imply that i have change LAN IP thus the error code 26106 happen. If I make a fresh setup without altering any IP, how can  not in the same subnet and previous one become a problem. What is previous setting that trigger the 'not in the same subnet' error. Are TL-WR1043ND still carry setting from DIR-616 made by TM.