MacBook Air was switched off for more than 3 months, now unable to connect to internet or update date/time.

If your MacBook is not connected to the internet, how did you get the message that the coverage has expired? When you click the wifi symbol in the menu bar, does it show any wifi network names? That is, are there any networks available?

If there is a network available, check to make sure you have the correct password. You may also be able to set up a wifi hotspot from your phone and test to see if your computer can connect to that. These instructions are for iPhones, but other phones are probably similar:

How to set up a Personal Hotspot on your iPhone or iPad - Apple Support