cant connect to firewall web UI timeout issue

[Zenhusen]

We have an issue with our firewall, since 3 months back everything behind the firewall is almost no accesible vi Internet, if we can access the different devices it is really slow . 
After some investigation we figured out it must be the firewall. So i connected an ethernetcable between my ps and firewall.

Did it as this

 On your management computer, configure the Ethernet port with the static IP address 192.168.1.2 with a netmask of 255.255.255.0.

2.  Using the Ethernet cable, connect your computer’s Ethernet port to the FortiWeb appliance’s port1.

3.  Start your browser and enter the following URL:

https://192.168.1.99/ 

(Remember to include the “s” in https://.)

But i cannot access the web UI, i get Timeout

How can i solve this issue,  I dont want to reset the firewall because dont remember the setting for it. 
I spent hours setting it up:-)

Hope for some helpfull input

[HarryTran]

The default management IP address is 192.168.1.99, but this value often changes once the firewall has been configured.
To locate the current address from the LAN side, try identifying the device that serves as the default gateway—this is commonly the firewall itself.
If you still cannot access it through the network, the most reliable option is to connect via the console port and check the management IP directly from there.