This is from my 86U Administration tab. It implies that an AiMesh router in AP mode ("AiMesh master") and AiMesh nodes can be connected to another router (ISP router):
o Wireless router mode / AiMesh Router mode (Default)
x Access Point(AP) mode / AiMesh Router in AP mode
o Repeater modeMedia Bridge
o AiMesh Node
"AiMesh Routers in AP mode connect to a wireless router through an Ethernet cable to extend the wireless signal to other network clients. In this mode, the firewall, IP sharing, and NAT functions are disabled by default. You can add AiMesh nodes to form an AiMesh WiFi system to provide extra WiFi coverage.
When AiMesh Routers are in AP mode, the DHCP-assigned IP address changes. Install and use the Device Discovery Utility in order to detect the wireless router's IP address."
ISP >> ISP router (firewall, only DHCP server, NAT) >> AiMesh router in AP mode >> AiMesh node(s)
OE