Electronics are not built to constantly be running at all times, and power cycling helps with most issues. Maybe the router is reaching its limit of ram.

have you rebooted after after you disable IGMP? if no luck, it is better to email tp-link support.

move it a little bit close to your router, range extender should be placed where it can receive good signal from main Wi-Fi. If you still have troubles, it's suggested to contact TP-link email...

nice post

maybe your Pi just wan't take that assigned ip address, try another one or contact Raspberry company for more help. Or settle down with dhcp

haven't try 4-5 voip phones. but you can send a email to TP-Link support maybe they can tell us