Hi. I am using bell fiber internet -1 gbps (modem +router 4000)

Whenever I add dns ip of either pi hole or adguard running on my home server, it will run its queries but then it will stop the internet. I tried adding in my router settings dns too nothing helps. Any idea what should be doing now,?

Ps- adguard and pihole are running fine

My friend suggested I should get a new modem but is it worth it?