Yeah, if thats the case, it sounds like everything on your side is correct in my opinion.
For good measure, I'd factory reset the router again, connect to the WAN port, and login to get mapped, and test it out.
It should work, and if it dies again, leave it in exactly this state and get hold of
@AfriNatic and
@Afrigirl for some help in further steps.
If there is
insane multi-cast flooding like Afrinatic said, that is always ongoing, that could be it, but I doubt it'd be this severe unless something is terribly wrong.