How do I access my modem's GUI when it's in bridged mode?

I am appalled at the misinformation in the accepted answer. While I have no experience with the ActionTec P5001a, I do have experience with half a dozen similar devices, and they all work the same way when in bridged mode.

When it's in bridge mode it has no web interface, no IP.

Wrong. It has the same default IP address (192.168.0.1) and the same web interface is waiting to be used. However, there is no route to the network, so you cannot reach 192.168.0.1 unless you tell your router to route to it.

I have installed DD-WRT on my Linksys E4200 router. It is available for the E2500 as well. Make sure that the E2500 network is anything but 192.168.0.1, then point your web browser to the router's management UI.

• Click on Administration

• In the Command entry field, enter

  ifconfig `nvram get wan_ifname`:0 192.168.0.2 netmask 255.255.255.0

• Click "Save Startup"

• In the Command entry field, enter

  iptables -t nat -I POSTROUTING -o `nvram get wan_ifname` -d 192.168.0.0/24 -j MASQUERADE

• Click "Save Firewall"
• Reboot the router

You have just given the router the IP address 192.168.0.2 which is on the same network as the bridged modem and instructed it to route packets to that network. From any device on your network you can now log into the bridged modem's UI with

http://192.168.0.1/

Managed to find my GUI in new address: http://192.168.100.1 (original was 192.168.0.1 before bridged mode) logged in and turned it back to router mode and after waiting it to reset Clicked network Internet access logo on desktop and there troubleshoot problems.

this video helped me: https://www.youtube.com/watch?v=C-0VhTXxHS8

In case of Arris routers (such as the DG1670) when in bridged mode the UI is available at:

http://192.168.100.1:8080/