I am getting buffering on both my 4k firesticks as part of multiroom setup. My service provider blames the ISP and I have done all the normal things like reboot the router/ firestick etc. I use other devices (laptops , phones) which don't seem to have a problem with the network and would like any advice as to how I can identify the real problem. Been with my service provider a couple of years and sub due for renewal soon. TIA