Password is usually case sensitive. If there are letters in last 8 digits of the serial number. 
 

I doubt the UI is locked down by ISP but just incase, you can try disconnecting coax cable, factory reset by holding down the rest button for 20 seconds and try the default logins. 
 

The SB8200 is a cable modem and doesn’t have advanced settings. What are you looking to configure?

Yes I have tried the reset. Still couldn't log in. I need to log in so I can set password. 

password for what? 

The SB8200 is a modem only device. It has zero routing capability or wireless. the only password would be the login and if you can’t even get it, neither is someone else. 

Yes you are absolutely correct. I don't know why I didn't realize that. I guess no one could log on to it because like you say it is strictly a way for internet to come into home. The only thing they can hack is my router correct? Thank you

For the most part yes. 

Things could always change but good security with your router is the key. 

how do you configure wan aggregation if its not in the gui  please help im confused now ( more than before lol)

shooties. You do need to log into it to setup wan port aggregation. Its not an auto-negotiation. 

Last option would be to disconnect the coax from the modem and with it powered on, try a 30/30/30 reset. 

thats 

while powered on hold reset for 30 seconds, 

power it off while still holding reset for 30 seconds. 

power it back on all while still holding reset for 30 seconds. 

Then see if you can access it