If you are unable to login to the modem GUI by incorrect password error the there is no option to reset or forget password on the modem interface so only option is to perform hard reset on the modem manually to login GUI using default login credentials. But after resetting the modem will go to default setup any custom settings will be removed so use the default network name and password to connect to your network.

http://arris.force.com/consumers/articles/General_FAQs/SBG10-Web-Manager-Access

Thank you.

What would be a custom setting on the router...security cameras, printer, PC?

Just trying to figure out what custom settings I’d lose and have to reinstall. 

To connect the devices, such as security cameras, printers and PCs, I would suggest changing the security mode on GUI web portal.

Login the web portal in any browser, such as 192.168.0.1

Login with credentials. Basic setup>wireless 2.4g> security mode as WPAWPA2-PSK (TKIP/AES)

Thank you