Wireless WAN throught switch port

Hi everyone,
I have some questions that I cannot find the answer else where so I make this post to ask for you guys some help.
I’m a noob about networking and I dont get much about networking terms so please straight to the point and terminal code, that would help me alot.
In the attachment is the diagram of my network.
I want to use the internet connection on the ISP3 to make 3 WAN load balancing at the RB1100AHx2. I already did the PCC load balancing with the ISP1 and ISP2 then it’s not the problem.I just cannot show the RB1100AHx2 the ISP3.

Thank you for reading this.
diagram.jpg