Update: In more recent routers and firmware versions, MTU can be changed in the router’s Web UI under WAN >> Internet Access, then click “Details Page“.
Click Start > Run and type Telnet 192.168.1.1 in the Open box as below. Note that the IP address in the example is the default address of the router. If you have changed the default, enter the current IP address of the router.
Click OK. The Telnet terminal will be open. If an administrator password has not already been assigned, follow the on-screen instructions to assign one.
After assigning a password, type . You will see a list of valid/common commands depending on the router that you use.
Type the command: wan ppp_mss then press ENTER (the current MTU size will display)
To change the MTU size type : wan ppp_mss 1500 then press ENTER (Note that 1500 is the MTU size).
Check the new MTU size by typing wan ppp_mss then press ENTER.
This command allows you to adjust the parameters for TCP protocol, MSS (maximum section size) for WAN side. The more the size is, the more the packets of payload will be.
wan ppp_mss [MSS size]
MSS size It means to set suitable size for MSS. The available range for this
setting is 1000 to 1500.
> wan ppp_mss 1442