A local charity asked me to have a look at their system as they'd lost internet connection. They have an EE FTTP installation, and there had been a contract change to change account holder names and billing details. The changeover was supposed to be transparent to the kit - i.e. no change of username or password for the connection etc.
Coincidental with the day the changeover occured, sure enough the connection went down. After a bit of poking around I worked out what was happening. There is a BT openreach modem (ECI B-focus 1B) connected to a Ubiquiti UniFi security gateway, and thence to a unifi 24 port switch. The modem had all green steady lights, but no internet connection showing up on any connected equipment. Bypassing the Ubiquiti box by taking the WAN connection from the modem through a spare TPLink router and from there to the switch gave everything connection to the internet. So the modem is connected and seeing the outside world, but the USG is not happy to pass things through. What change is it likely seeing that stops it allowing traffic through?