While many global routers use 192.168.1.1 or 192.168.0.1 , Viettel modems often use specific addresses depending on the hardware model (Huawei, ZTE, Gitron, etc.).
If 192.168.1.1 does not work, check the physical label on the bottom or back of your Viettel modem. It will list the 192.168 viettel