Mikrotik Routerboard 450G Dual WAN Problem

Hello, I am very new to miktotik routers and bought Routerboard 450G to user 2 ISPs at the same time, One has 15MB speed but the ping is very bad, The other one is 5MB and ping is very good and 100PCs can play without any lag. I succeeded on configuring both of them but the router uses only one of them when both of them are plugged. The objective is to have 2 PCs use the 15MB ISP and all the other PCs to use 5MB ISP under the same network all should be able to connect each other. OR can I combine their speed and still get the lag free ping?

https://wiki.mikrotik.com/wiki/Manual:PCC


Yours respectfully!