Here are some important steps to go by after the filter is installed on Android or iOS before the customer leaves the office, always making sure that all works:

Make sure the device has 4G/3G Data working, and can browse the Internet.
Make sure the device has WiFi working, and can browse the Internet.
Make sure the filter is working on both connections (4G/WiFi), try a site that should be blocked to see if it actually gets blocked.
If something is not right try to solve it on your own before the customer leaves, or call support for help.

Things that may cause no Internet on Android devices:

Make sure APN settings are set to IPv4 (only)

IPV4