after you downgrade the firmware, it works fine on those smart phones? for the pitch noise, you'd better contact tplink technical support

If you have changed the wifi password but forgot it, you can connect a computer to router via ethernet cable and access the router web interface to find or change the wifi password; but if no any...

what is the model number of your router?

I think you need to contact those ISP to confirm the speed they can provide

only power light on? no wifi light? if there is a wifi button on the back panel, you may try to press it to see what happens to the wifi light and you may also try to reset the router to factory...

what is the hardware and firmware version of your router? you's better send a screenshot of your settings as well to make it more clear