experiencing irregular drop out -router? settings? can router help diagnose source

Hi I have had our internet supplier investigate the drop out issues. They have installed a monitor onto our home network that has told them its not the fiber supply that is the issue, it is something our side. The drop out occurs around maybe every 8 mins for couple of seconds though sometimes it can be ok for half hour or more. The network is hard wired (10/100) to 3 high spec pc's and sky Q plus wifi to phones etc.  We have 2 wireless network links (through mains) also, though these only extend the wifi. Its the hardwired PC's and sky Q we notice drop out on mostly. 
The internet provider tells us the issue happens when we swamp the router with simultaneous uploads and downloads. That is when both are at max at same time it drops signal. My question is can the router help me narrow down which device is causing the issue, or help me limit a device to narrow down or fix the overloading. Or could it be the router is not high enough spec for our needs. We have a Zyxel XMG3927-B50A. This was supplied by our provider. we get around 400 upload, 40 download on speedtest.