AC68U - Trouble on assigning fixed ip and using Adaptive QoS

[debtif]

I am not a geek, but after research and many failed attempts, I was able to flash Asus Merlin firmware and setup two routers to share internet with my 4 neighbors in the same household 1 house down.

My setup
Main router - RT-AC68U firmware 380.64 (IP 192.168.1.1)
Repeater - RT-N66W firmware 380.64 (IP 192.168.1.2) - 4pcs (PC1 to PC4) connect via wifi to repeater

My internet have fine. But the main problem is when my neighbor PC1 or PC2 is online, the other 2pcs (PC3 and PC4) internet speed slowed down to a crawl.

Since all neighbors 4 pcs showed up under System Log > DHCP leases with their own ips, I was going to use the Qos to fix the issued, but...

Adaptive QoS > Bandwidth Monitor showed Repeater - RT-N66W only. The 4 pcs isn't there at all. So I can't set priority individually for PC1 to PC4 here.

Same with Adaptive QoS > Qos > QoS Type = Bandwidth Limiter, Client List only contain Repeater - RT-N66W. So I can't set limit download/upload speed for PC1 to PC4 here either.

I tried manually setting the IP under Lan > DHCP Server, but they still won't show on Adaptive QoS and Bandwidth Limiter list when they are online or offline.

My brain is fired. Is this a bug that any pcs connect through repeater won't show on the OoS Bandwidth Monitor and Bandwidth Limiter list or any menu within Asus Merlin firmware? Is there anyway to solve this issue?

Thanks a million.