Thank you so much! This works! It seems so obvious now.
I did already try accessing the router with ESET completely disabled but it still didn't work (I assume it's still running in some form because it's blocked other things before when *disabled*).
I think it threw me off that it wasn't an issue with my machine as it happened on my phone too but I think that might have just been a temporary glitch. It definitely only started happening after Windows 8.1 though so maybe there's something about the two that don't sit well together?
Thanks again!