I would first try a new router.
Then maybe a WiFi extender.
Cell use on WiFi?
Sure you can.
Flip the shiny switches!

To turn yours on, go to the Settings menu. On iPhones go to Settings > Cellular > Wi-Fi Calling and then toggle on Wi-Fi Calling on This Phone. On Android, you'll generally find WiFi settings under Settings > Networks & Internet > Mobile network > Advanced > Wi-Fi Calling, where you can then toggle on WiFi calling.

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