@Anirbandt 

Hi,

I don't know why things worked fine with the old router and not with the new router, but to me it seems this error message points to a subnet mask that is set too small.

For example, a subnet mask of 255.255.255.192 allows a certain block of 64 consequtive IP addresses. If this subnet mask was used in conjunction with the two IP addresses 172.20.0.109 and 172.20.0.1, then this would not work, because 109 and 1 span a range of 109 addresses, while the mentioned subnet mask only allows for 64. The closest subnet mask that should work with 172.20.0.109 and 172.20.0.1 would be 255.255.255.128.

Without knowing your actual IP addresses I can't come up with a suitable subnet mask for your case, but you can do it easily yourself using an online subnet calculator, for example this one.   www<dot>calculator<dot>net/ip-subnet-calculator.html   (replace <dot> with .)

  @woozle Yes you are right my subnet mask is 255.255.255.128 and ip is 172.20.186.109. but still it's showing that error.

My service provider gave me a new set of static IPs. it's not giving me any error when I am putting it in the router static settings but after putting nothing happened. in my network its showing connected to a local server without internet.

  @Anirbandt 

Have you also tested the new IP addresses from your ISP with the old TL-WR740N router, just to confirm they are actually working?

By the way, what kind of subnet mask is used shouldn't actually matter, I think.

If the C50 for some reason doesn't want to take the old set of IP addresses together with a subnet mask of 255.255.255.128, then just try 255.255.255.0 or even 255.255.0.0.