Games : PS3 connects to Wi-Fi but fails to sign into PSN?

I have two modems. One that is an ADSL and the other one is a Wi-Fi.

My PS3 works fine when I directly connect the ethernet cable from the ADSL modem to it, but whenever I try to connect through my Wi-Fi, it keeps failing at the PSN sign in.

This is my Wi-Fi router - http://www.tp-link.com/en/products/details/cat-9_TL-WR740N.html#specifications

I already tried disabling Media Servers. Am I supposed to be modifying any settings, such as MTU or uPNP? Or is there an issue with my Wi-Fi modem?