Hello, friend.
If you want to configure HG8245Q as a bridge for a second router, You could refer to the methods provided by Wissal.
About the failure to add a new WAN configuration, you may not have sufficient rights for logging in to the ONT. You need to log in to the ONT as an administrator and then configure the WAN. For details, see the WAN configuration Figure 1.
Figure 1

NOTE: If your ONT is customized by an ISP, contact your local ISP to provide the administrator account and password for login.
Thanks.