At random times my Archer A7 router starts routing everything but local network back to itself. By that I mean every IP address I ping will be that of my router. If I visit any domain it will tell me "this website is not secure".

As it happens at very random times (once every three days or so), I don't know what I can do to obtain more information to figure out what's going on. All I know is that it started happening after my ISP reset the (fibre) line (though could be unrelated), then disappeared for a few months, and now it's back.

So my question is: has anyone else have had this issue? What can I do to debug this?