tracert request timeouts

[Caple]

Hey everyone.
A friend of mine asked me something and I couldn't help him.
He has two internet connections (1 ADSL and 1 cable) connected to a TP-Link dual WAN router. On that he has a PC, a NAS, a printer and another router (let's call it router X).
The computers connected to the router X, when using the tracert command, always receive a timeout message, but are able to reach the destination.
The inclued image helps to explain. It's in portuguese, but you will understand the situation.
I couldn't find the reason why its acting like that.
Can anyone explain to me?
Thanx.

 

[System Error Message]

Its probably dropping ICMP packets. Totally fine and in some places is done for security. Just make sure that there is no double NAT. Setting ARP to proxy may solve the problem.

 

[Caple]

I was suspecting of double NAT, but totally forgot to read about it before posting here.
Thanx for the reply.