How do I access the modem once I put up the firewall?

I hope this question isn't too basic, I'm new to networking, no certs, and just getting my first experiences with it.

Basically I'm wondering how this works.

If I install a firewall it's ip is 192.168.1.1:8080

I have a watchguard t15 firewall

What happens if I need to access the modem/router again, because now the firewall is the new default gateway at 192.168.1.1.

The modem/router is a cisco model I can't remember the name.

What is the IP of the modem now? How do I get back to it's back-end? Do I access it through the firewall?

Another thing I thought is if I just unplug the the firewall from the modem/router then plug in my computer would that solve my problem and I'd be able to connect to the back-end.

If I plug in my computer directly to the modem/router will it now become 192.168.1.1 again and the new default gateway.

At this point I will have turned the modem/router on pass through so through traffic passes through it, and the wifi is turned off.

I'm really sorry if this is to basic I tried to google search it before I posted.