I'm sorry I take the access point as 2.4GHz...How about updating the firmware of the access point to the latest one?

I know the speed would reduce when it is going through the router's USB port, but I'm not sure what is the speed it can be:( It is said that only 2 users can access the FTP sever at the same time.

I'm not so sure what the settings you have done. Can you post a picture of your routing settings? I will see what I can help:)

How about fix a channel such as 1, 6 or 11 on the AP router?

I'm a little curious about the spin down capability. How is that working?:confused:

In that case, I think you can set the load balance, making a rule that data from the IP 192.168.1.5 only go to WAN 2.