I also have Beetel 777VR1 ..... had same issue with this modem when i enabled G.INP
can u check if G.INP is enabled or disabled on your modem ?.. (to check if enabled or disabled click on setup -->WAN-->DSL--> DSL Modulation: G.INP ) .... if enabled try to disable it and check if u can get stable connection ?
and also enable logs so u can know why its getting disconnected and how many times your WAN got disconnected ( to enable logs click on maintenance --> Log ->Log-> check Error and Notice than click apply changes )