Hello Phil,
From what I can see on your answer, it seem like you did all the posible options to diagnose the issue with the tools that you have around. You did it well but still I do not understand why the clients are without internet.
At this point, I will let the support team from UPC solve your issue, I am sure that they will check if you have the proper signal, and that the account is working as expected in their system.
I am sorry that I can’t help you more.
About the firewall and NAS, if it was working before then it should work now.
Have a nice rest of the week.
Elías