I don't know how the IP route in MT works , and how some client react on redirects.
A more detailed and more correct wording.....
"The subnet is used to determine what hosts are "assumed to be on link". If a host wants to communicate with a host outside it's subnet then (assuming there is nothing special in the host's routing table) it will try and send the packet to it's default gateway.
If the default gateway knows about all the subnets and is prepared to send packets back out on the same interface they came in through then communication will work fine. Otherwise things are going to break.
When the default gateway notices that it is sending a packet back out the way it came in it may send ICMP redirect packets to inform the host that it can send the packet by a more direct route. The sending host may or may not take notice of said redirect."
Source: https://networkengineering.stackexchang ... e-same-lan
It's about having multiple subnets on one LAN or not. If one splits multiple subnets over multiple ethernet ports or over multiple VLAN's is the same avoidance of multiple subnets on one L2.
Using VLANs requires at least one edge port per VLAN, if client devices are not VLAN aware. In that case with just one router the same splitup happens if the ethernet port is taken of the bridge.
OP only mentioned a hAP ac2. Device interconnection is unknown. More edge ports could be created with a VLAN aware switch.