I have my phone connected using the password on the back of the modem. I'm trying to connect another phone using the same password. It won't connect saying invalid.
Make sure the characters are being input correctly. Like make sure an I isn't a 1, or vice versa. If it worked with the one it should work with the other.
You an also sign into the WiFi section of the modem to check the password(s) shown for the two bands (2.4GHz and 5GHz), which should be identical unless they've been changed. You can log into the modem's WiFi settings via the following link...
Use admin as the password for that access.
I see it's your first post here, so welcome to the community! I hope Gabe's suggestion put you on the right track. If you still have concerns, feel free to drop by the community again!