router password not set to default

I just had a new router installed today.  The technician said the admin password would be set to the password I had set previously.  Everything was done and about 5 minutes later, someone accessed my PC remotely, put up a screen that said "software updating, do not turn off" and then the mouse pointer started moving around wildly.  I was sitting right there and saw this happen, so I immediately turned off the machine.  I turned it back on and it was on an amazon page with a $50 gift card in the cart.  Nothing had been ordered.  I did change the password on Amazon, but since they got there using my browser on my machine, I suspected someone must have come through my router. 

I tried to log into the router with the admin password I had previouslly.  It was not the right password.  I tried the default password.  It didn't work.  I thought maybe the tech set it to my wifi password, so I tried that, no luck.   I really don't think the intruder had enough time to reset my router password, but it may be a possibility.  How do I get a router password that I actually know at this point?  

BTW,  Xfinity needs to know how absolutely successful the Xfinity Assistant is at being frustratingly useLESS, but nothing more.  Trying to get support from Xfinity is absolutly maddening!  If this is the best support I can get, then they should be paying the forum users!