Many websites not opening from a certain WiFi connection
I have had similar issues in past and suspect this is because of the WiFi connection.

It is most likely an issue with the Router's/Modem's MTU size settings and not necessarily because of an issue with your MacBook Air.

There are many helpful links online on what settings you can try with. One which might give you some insight is here- https://serverfault.com/questions/70075/why-does-lowering-the-mtu-from-1500-to-1499-allow-me-to-access-most-websites

A quick suggestion could be to compare the MTU settings of a working router with those of the non-working one.