Missing router password; best or quickest solution? HELP !

My landlord signed up for a new AT&T Internet plan giving double the speed.

Tech was here yesterday. Obviously replaced router with one able to handle the new capacity.

Now I can’t get on to WIFI. His computer is hard wired connected and he claims that he can’t find his password.

So, I’m assuming that either the AT&T tech or the company that made the router have a method for resetting to default enabling to choose a new password. Is this correct? What’s the easiest way to accomplish that.

I’m not really even sure that he changed from the default or if his computer had the password stored in memory and automatically applied it to the new installation.

I’ve tried obvious stuff like tech1234 or admin1234 or password or administrator but nothing obvious like that worked.

So what’s next? Any advice gratefully received.