The simplest configuration would be to configure the AX3 (hap ax3?) as an access point, and not as a router. Then everything would be in the same network provided by the LAN side of the RB4011, and the hap ax3 would just be acting as a wifi access point with 4 available ethernet ports configured as a "switch".
If you want more help, you are going to need to provide more information.
Please provide a network diagram (even a photo of a hand drawn sketch) with how things are connected, the ports being used to connect on the RB4011 and the hap ax3. Also provide exports of the two routers.
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