xfinity gateway password change

Installed a new xfinity modem/router. Logged on to default gateway from a computer with default credentials. It prompted me to change the password, but when iIclick 'save', the operation times out. Always! This is the 3rd device in week I've installed as the other 2 stopped working after a few days. But, they all allowed me to change the gateway default password. Model is CGM4140COM.