Changing DNS on Tenda D301 modem

How can I change the PTCL auto-allotted DNS in Tenda D301 modem to a DNS server of my choice such as 8.8.8.8? It seems PTCL has disabled access to DNS settings menu so that you only use their DNS server. Any change in DNS that you make is not reflected in the settings or stored.

Look into using telnet