404 Error after electricity interruption

I purchase my Orbi in 2017 set up and everything has run smoothly until last week.  In the middle of the night our electricity went out and back on approximately 10 times within 30 seconds.  Then electricity went out completely for 3.5 hours.  It did another quick on-off and then came back on and stayed.  The next morning I had to reset my ATT modem no the Orbi.  We could use the internet but on a lot of websites would receive a 404 error and some apps would say there was no internet connection at all.  Also some web sites I get a message say that the server is asking for my username and password.  I did a second reset and everything seemed to be fixed.  Then yesterday am, another quick on-off of electricity and now I'm right back to the problems.  2 weeks ago ATT installed a new modem.  Could there be a problem with it.  What can I do to stop this?  I have the wi-fi disabled on my ATT modem.  I appreciate any help I can get!