Help, Wifi gets disconnected from Internet frequently!

My problem is regarding my new D-Link DSL-2750U ADSL Modem cum router. I have a telephone line broadband Internet connection (I live in India). My wifi is getting disconnected from the internet regularly, and internet doesn't work until I reboot the router cum modem (Wifi connection remains but it gets disconnected from internet). This happened with my previous ZTE too and I thought it was a problem with that modem cum router so I replaced it but its happening with DLink too. I thought it might be a problem with my laptop but even my phone does the same thing. I read on some websites that having too many devices connected can cause problems, but I have only my laptop and my phone, and laptop disconnects from the internet even when phone is switched off for long time. How can it be a problem with the connection (line), since when I reboot the router it starts working again. I also tried changing the wifi channel. I am sitting very close to the router.

Could you please help me? Thanks a lot. Any help will be appreciated.