you have to be more specific of what you are trying to achieve.
the bridge mode, does not necessarily mean that your ISP_ROUTER will only act as modem without log into your ISP_account:
Bridge mode is a special mode of operation where the current gateway/modem acts as a network bridge, forwarding all traffic to a downstream device.
If you will be using your ISP_Router as a modem only, and you want to add your ISP credentials in your OPENWRT_router, the best guide you will find in the following article:
ISP Configurations
hint: try to find the guidance about Greece, our providers use almost 90% Speedports for equipment.
In other scenario, if your ISP_Router will do the handshake and also log into your ISP_account, but the rest will be done from OPENWRT_router, read the following topic:
How to redirect wifi traffic from ISP router through my own router