Android devices behind a Guardian web filter cannot access the Google Play Store.
The Google Play store application uses port
5228 and does not go through a web proxy.
In order for Android devices to be able to access the Google Play store, outgoing access to TCP and UDP port
5228 will also need to be allowed from the network the Android devices are connected to.
However, there are multiple applications in Android and iOS that do not use web ports and protocols in order for those to work correctly, outgoing traffic on those ports will need to be allowed from the network the devices are connected to. It is recommended you contact the applications' support in order to find these ports to add these to your firewall allow rules.