Make sure the LAN IP address subnet of the bridged modem is different from LAN IP address subnet of the router. For example, if the router IP is 192.168.1.1/24, the modem should be on 192.168.10.1/24.

Add another IP address on the WAN side of the router. This router IP address should be in the same subnet and different address as the modem.
If your modem is on 192.168.10.1, use this command: /ip address add address=192.168.10.2/24 interface=ether1. This will add another IP address of the router to 192.168.10.2, allowing you to access the modem at 192.168.10.1.

Now you can access the bridged modem behind the router by going to 192.168.10.1.