What Hardware version is your router? Look at sticker under the router case.
Link>What Firmware version is currently loaded? Found on the routers web page under status.
What region are you located?
Can you access that IP address of the modem from your PC browser when connected to the 868L, you should be.
What IP address is the WAN port getting on the 868L? It should be a public IP address of ##.##.###.###. IF your ISP modem is bridged, then the 868L should be getting a public IP address and not a private address 192.168.###.###. This is preferred to attain single NAT conditions. You wouldn't have to port forward anything from the modem or it's IP address.
Again the modems IP address of 192.168.100.1 is only for the modems web page and not used for anything else.