It is very likely that your gateway does not respond to ICMP Echo Requests, otherwise known as ping's. It's actually quite common for this to be the case, and must be in this situation.
I've found these documents which reference a Web UI, as well as other access methods.
Translated specifications sheet
Untranslated user guide
If you cannot connect to the gateway on the internal LAN on either port 80, port 443, or port 8080 (http://[gateway-ip] or https://[gateway-ip] or http://[gateway-ip]:8080 or https://[gateway-ip]:8080) then it is most likely in a bridged mode. You might try resetting the gateway and see if you are able to access it then. Bear in mind though, any configuration will be lost after a full reset. An alternative would be to run a port scan and see what ports are open - if any - and this should help in finding the access ports.
If the router was in a bridged mode, however, you would not see a gateway address for the device or be receiving DHCP from this device, but it is possible you could be seeing this information coming from an upstream device - such as an ISP's gateway which could be handing out private IP addresses - again, very uncommon, but possible.
Of course, all of these things would be dependent upon your current LAN setup and your ISP's routing configuration. Unfortunately, the only way of truly knowing how to access and configure your gateway/firewall would be to have readable documentation on the device. The previous spec's sheet and the untranslated user guide were the only documentation I could find, unfortunately.
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Neither ping or gui access are needed for the router to provide access to the internet. Are you saying you were able to access the gui previously? – Paul – 2013-07-09T04:45:37.950