Wireless(wifi) is not the same as mobile(cellular). Does the phone show the icon for a cellular connection along the top? It will be separate/different for the symbol for wifi. If not, try going outside or moving around to see if it reconnects. Also go into settings and make sure the cellular/mobile connection is activated. What kind of phone do you have for instructions? Last, you can turn on wifi calling, which should send the phone call over the wifi connection instead of cellular. This should only be a short-term fix as you pay for your cellular bandwidth so it should work.