I don't know what I do or what my ISP do some changes. and now I have to change my computer network adapt's mtu to 1492. and my computer can open website. otherwise I can only open few websites. do you guys know how can I do in my cisco 1941 routers for without changing computer mtu? I have tried change router diale1 mtu to 1492, but it doesn't work.
1) You should set the mtu on your WAN-interface. Depending on your ISP-connection, that could be one of the Gig-Interfaces or a Dialer-interface:
interface Dialer 1 mtu 1492
2) Configure the router to reduce the maximum segment size (can be on the LAN-Interface):
int gig 0/1 ip tcp adjust-mss 1452