I have installed ubuntu mate 20.04 on my lenovo laptop. Wifi not working. the modem is Realtek one. its showing on the list of networks but its showing disconnected.
Hi 
Can you try the wireless radio key combination on your computer (like fn + F5) with hope it's only radio disabled ?
I have checked. For my system (fn + f7) combination only toggles bluetooth. it doesn't turn-on/off wifi. There is no other combinations.
arfff 
Can you try this command to see the result ?
/sbin/iwlist scan