I see the picture you included of the bottom of the unit. It shows the user is TELMEX. Is TELMEX your ISP? It could be that this unit was manufactured for TELMEX, and that the manufacturer used custom firmware that may not let you change the DNS address. There is another way around this problem. You would need to change the DNS in each PC you connect to the router in order to use a different DNS. This can be done in Windows, even if your PCs are using DHCP for the IP addressing. The DNS setting in each PC overrides the one the router uses. So that may be another option for your situation.
Hope this helps!
....JIM....