Hello,
Thank you for contacting Rank Math today.
Well, that function is not offered (yet) by Rank Math, you can set as the fallback URL the page with the 404 error explanation and all the 404 errors will be redirected to that page but they won’t registered in the 404 monitor since they are being redirected somewhere (so they are not 404 errors anymore).
I have attached a screencast showing you where you have to set the custom 404 page URL: https://i.rankmath.com/eInhe5
Looking forward to helping you. Thank you.
​​​​​​
Hello,
Since we did not hear back from you for 15 days, we are assuming that you found the solution. We are closing this support ticket.
If you still need assistance or any other help, please feel free to open a new support ticket, and we will be more than happy to assist.
Thank you.