How find network password when information on label on bottom of router/modem is wrong

Total newbie here, so please forgive dumb questions and lack of even basic knowledge.  I purchased a Nighthawk AC1900 WiFi Cable Modem Route for Xfinity Internet and Voice, Model C7100V last year because I wanted to save money and quit paying Xfinity rental fees.  When I got the modem I called Xfinity and they helped me set it up.  Everything worked fine.  Now I have purchased a Firetvstick.  When I try to set the Firetvstick up it recognizes my network but the network it recognizes is not what is on the label on the bottom of the modem.  It wants a password for the network and I do not know that. The label on the bottom of the modem shows a network name and password that do not work.  Even the default access information on the label does not work.  How can I find out what the password is for my network?  I've seen that other solutions posted here recommend resetting, but I am completely ignorant of what that might do and am afraid of losing my Xfinity access.  Can anyone be so kind as to walk me through a solution for this?  Thank you very much!