Cannot access OWA URL or send Emails using Activesync when connected to Wifi network

Our Wifi network used to work fine when accessing our Exchange OWA URL or sending emails from devices that used ActiveSync connected to the Wifi. We can access all other websites fine and emails also work fine on 3G/4G and other wifi networks.

For some strange reason, this has now become a problem.

The only Wifi change that has happened recently is changing of the Wifi password but I don't see how that would affect access. I have already removed the Wifi network from devices and added it again. But this has not fixed the problem. We still cannot access the Exchange OWA URL nor send emails from devices.

Any ideas?

Devices connected to Wifi cannot access the Exchange URL?