If your router does not allow you to customise your DNS settings in any way, you can still try to have Pi-hole distribute your local DNS settings by configuring Pi-hole as DHCP server.
For that to work, you have to either disable (preferred) your router's DHCP server.completely or limit (second best) your router's DHCP range to just accomodate Pi-hole.
As this is highly device-specific, you'd have to consult alternative superior sources of knowlegde like your router's manual or forums for advice on how to achieve this.
Also, a perfunctory search here on the forums brought up How to set pihole as DNS on Arris TG1682G router (Comcast)?, which is not exactly your model, but might still be relevant - have a look, and feel also encouraged to start your own search 