Unknown device logging onto network

Just registered my EX6100/AC750 Dual Band WIFI Range Extender after going through the entire setup process...step by step. SUCCESS! I unplug it and plug it into the next room so I can connect my IP phone for work & the light turns red! So I unplug everything & take the extender back in the other room & reconnect it to the router (Nighthawk AC1900). I try to log into mywifiext.com and it's saying the password I created is wrong. Yes, the one I just used 2 minutes ago. And yes I double checked spelling/capitalization. So, I try to contact support, but its asking for the serial #. I enter the SN and it says "Please enter a valid serial number". OMG, I'm loosing my patience! Please help. Thank you.