Question Fibre Modem - Watchguard Preventing Configuration!

Apr 7, 2019
I have a LAN which is protected by a Watchguard (Firebox) and access to the internet is through a standard, fibre modem provided by Zen.

I know the ip and port number through which I can access the Watchguard for administration purposes. This is the same ip address that I get when using IPChicken through a local browser.

But, I believe that the fibre modem was given a slightly different ip address (Zen gave me a range of 10 numbers?) and the guy who set my network up is unavailable.

So this week (long story) a change was made by my broadband provider (Zen) which necessitates me changing the user and password on my fibre modem. But I cannot connect to it to do this and so I have lost connection to the internet.

My thinking is:
a) I can try each of the 10 ip addresses that Zen gave me - but what port do I need to address?
b) Can I connect a PC direct to the modem and access the admin console through the modem's default credentials?
c) Is there any way (using MSServer 2012 or the Watchguard) to identify the mdem's connection?
d) Should I just factory reset the modem, add new credentials and then fix its ip address again?

Grateful if someone could point me in the right direction.