Your problem certainly seems to be related to WiFi bandwidth. You could try moving your ISP's router - sometimes even a few feet can make a big difference in WiFi reception. But you can probably do a lot better using your own router.
You don't even have to put the ISP router in bridge mode. My current ISP gives me a combined modem+router where the router section is also locked down in certain ways, and they won't allow bridging. I just attach my own router as a device on the ISP router's LAN, put it in the DMZ so that all ports get passed through, and turn off the WiFi on the ISP router and use my own router's WiFi. You don't even have to disable the ISP router's WiFi really if you aren't using it.