My comment was getting rather long, so I will post this as an answer.
Tldr; The answer is "AIN'T GONNA HAPPEN.
I asked you in comments
Can you define "pornography", because, if you cannot then you cannot
block it? Good luck in not blocking too much or too little.
and you replied
of course that defining clearly, directly and unambiguously
"pornography" is rocket science! Though it should not be hard to a
have a comprehensive list of all websites with such content.
(these two statements seem to contradict each other)
LOLX! I know it when I see it - but can you explain that to your router?
As to "it should not be hard to a have a comprehensive list of all websites with such content" - I am astonished that anyone can be so naive. There are this many web sites (currently 1.2 billion) and more than one new domain is created every second. Who is going to look at those and decide if they need to be on your router's list?
To reiterate, the true answer is "AIN'T GONNA HAPPEN.
Now, if you care to rephrase you questions along the line of "how do I get my router to refuse to serve certain sites?", then you have answers above, such as that from @Nivatius
You could ask a separate question, maybe here, maybe on our sister site https://opendata.stackexchange.com/ requesting a list of "all the porn sites in the world". After the laughter dies down, you might well get a few lists, which might be helpful.
BUT, you state "I want to set myself free from pornography" - so, you have to accept that technology can never do that for you. It is a matter of self discipline (I am not trying to make that sound easy; I am just defining what it the only possible solution).
I don't know if seeing unexpected pornographic adds upsets you. If so, you can blacklist sites which show such ads, one by one, as you encounter them).
If it is a matter of self control, then accept that even if you did achieve what you wanted with your router (impossible as it is), you probably have a data allowance on your 'phone, and could use that, or you might find free WiFi in shopping malls, libraries, or even at work - which might lead to some embarrassing situations, and even more awkward questions on https://workplace.stackexchange.com/
Again - you will not find a technical solution.
Perhaps some other sister sites like https://lifehacks.stackexchange.com/ or even https://interpersonal.stackexchange.com/ , although this is not strictly interpersonal.
I hope that I have not come over as too hard. And, without knowing your motives, it is not easy to answer, but a router based solution will, at best only reduce exposure to pr0n, and can never eliminate it.