I think I've found the problem, after the advice of Stella when the reboot went right I watched the modulation assigned by the DSLAM so I changed to Advanced Setup -> ADSL the Auto Sync-UP specifying...

All routers and computers are connected on the same switch.

The topology network is very simple: 192.168.1.1/24 8840T PC1, PC2, PC3, PC4, PC5 gateway 192.168.1.1 192.168.1.2/24 8840T---> one switch TL-SF1024 --> PC6, PC7, PC8, PC9, PC10 gateway 192.168.1.2...

The problem is not in the resolution of domain names but the problem is that the router does not align properly with the ISP. I have asked the ISP support and they say they do not have problems on...

it seems impossible to keep 2 or 3 td8840T router on the same network after a few hours the network goes down, I believe to be the RIP protocol that I can not find how to disable, any ideas?

in the log of the device can not find any apparent problem, cetalfio 1/1/2000 0:0:1> Last errorlog repeat 1 Times 1/1/2000 0:0:1> SNMP TRAP 0: cold start 1/1/2000 0:0:1> main: init completed 1/1/2000...