If it's just local file transfers, I don't see how it would be your ISP. Would have to be the router.
Have you tried doing a hard 30/30/30 reset on your router? When you say you did a factory reset, I assume you meant you did this through the GUI?
A 30/30/30 reset is:
Hold the reset button down for 30 seconds. While STILL holding the button down, UNPLUG the router from power. Continue holding the button down for 30 seconds. Plug the power back in while STILL holding the reset button down. After plugging it back into power, continue holding for another 30 seconds. In total, you should have held the reset button down for 90 seconds without letting off of it. After the last 30-second hold, release it and wait for it to boot back up completely. Then power it off, and plug it back in. When it's fully booted again, log into the web GUI and reconfigure it.